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Advice needed on repowering a 5.8 tinny

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    Posted: 09 Dec 2019 at 12:03am
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Hi, I'm looking at repowering my 93 Alicraft Centurion and I like the look of Optimax, I had one for a short period on a 6.1 Bluefin and was impressed but didn't own it long enough to really get to know it. The boat currently has the factory fitted 115 Evinrude and I'm happy with the HP but want better economy. Any advice would be appreciated.... the boat has tote tanks
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Post Options Post Options   Likes (0) Likes(0)   Quote MacSkipper Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2019 at 5:58am
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When you say you want better economy you can buy a lot of gas for the cost of a repower are you happy otherwise with motor?
Are you looking at a new motor or used?
Is your present rig optimised ie engine at right height correctly propped running well.?
Being over or under propped can affect fuel economy by up to 50% or more and it is not uncommon for rigs to be poorly setup by dealers.
What is your present fuel consumption how do you drive boat what sort of load/ approx weight of boat gear and how many people are you running?
As a 5.8 m boat length is it a heavy boat for its size, how do you drive it ie general fishing cruising or towing kids toys etc?
Others with similar motors can comment on your fuel consumption then.
Good fishing trip nothing breaks, great trip catch fish.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2019 at 9:03am
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Repower..
1st thing look at the normal gross weight on the water..
 If you know the current top speed at best trim for that speed, on flat water,
 normal load,
 rpms between 4500 and 5500
 And engine height correct (bottom of cavitation plate in the water with a few drops rolling down the to surface like condensation down a window.

Then the weight can be caculated.
3 adults , gear I would guesstimate the weight around the 1200 to 1350 kg
If engine height correct about 37/39mph WoT
And a guestimate at correct prop about 17" pitch 14 1/4 diameter for correct slip numbers

115 being on  the min powered side to take full advantage of hull design and economy for a boat of that weight.
A 1250 kg gross weight on water, plaining trailer boat well powered is around 140hp.. upping to that ot even over power to 150 V6 you will pick up a good 10% economy over and above the 115 ASSUMING the 115 is set up and propped correctly

In most cases poor economy is a boat is between min and well powered is because of engine height and/ or propped to incorrect slip numbers.

A boat that size engine down 1 or 1 1/2 holes below norm will be about the equivalent of adding 1 1/2 to 2 plus ppl ave weight 90kg each
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Post Options Post Options   Likes (0) Likes(0)   Quote Mejiro Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2019 at 11:04am
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Thank you for the replies, I did forget to mention that one cylinder is well down on compression. The motor still starts and idles better than any similar motor I know of and runs well thru the rev range. It's possibly heavier than other tinnys as Alicraft were a jet boat manufacturer and from memory were 6mm hull and 5mmm sides and built strong. Top speed has been 29 knots, I think revs around 5500.
When I've been out its usually 2-3 pob and used for fishing with no heavy loads on board.
There are no visible numbers on the prop without taking it off so it could be under propped, it does perform beautifully in everything except top speed and economy.
It is a stainless prop and from a lot of reading on here the engine height looks right.
The last couple of years have been rough and two weeks ago I bit the bullet and advertised the boat, then I got a break, a small mortgage that reduces my debt repayments by 80% and allows me to upgrade a few things that need it and I think I'd have a lot more confidence  with a later model and more economical outboard.
Steps I can now see that more HP is a good way to go thank you.
68 year old pensioner so economy is much better than speed


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Post Options Post Options   Likes (0) Likes(0)   Quote Rozboon Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2019 at 3:06pm
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If the current motor is the same vintage as the boat, it wouldn't surprise me if you close to halved your fuel burn with a modern injected 4-stroke motor. The new motors return some fairly silly figures for fuel economy, especially at cruising speeds.

At the same time, the difference in price between a 2-stroke and a 4-stroke buys a LOT of petrol.

Many side benefits to a 4-stroke.... perfect starting regardless of temperature, quiet and smooth running, no fumes if that's an issue for people on board, no oil being burned, better for the environment if you care about that...


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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2019 at 4:53pm
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I did forget to mention that one cylinder is well down on compression.

 Thats going to hurt power and economy substantially.. not a good sign

It's possibly heavier than other tinnys as Alicraft were a jet boat manufacturer and from memory were 6mm hull and 5mmm sides and built strong.

 Accounted for that in my guesstimate of weight and from other boats in my data base

Top speed has been 29 knots, I think revs around 5500.
 Thats would be well propped assuming slip numbers ok..
When I've been out its usually 2-3 pob and used for fishing with no heavy loads on board.
 Yep basically normal gross weight on water
 Assuming that no power loss from the cylinder down which will have comes in around 1650/ 1700 kg gross weight on water...
 So think say 1700/ 1750 kg would be a fair ball park.

There are no visible numbers on the prop
 look at the rubber center peice from the rear.. will have a part number and something like 17 X 14 

lot of reading on here the engine height looks right.
 From the data supplied  height propping look as if has been set up well in the past

68 year old pensioner so economy is much better than speed

1952 was was a good yr Wink
Speed.. thats just an unfortunate side effect for having good power reserve power at cruise and lower aspeed to get thru chop without working the throttle..
 Have you taken the missus shopping basket out see how fast it will go? and for 10 mins?
 No? same with a boat.. same thing..

Regardles of 2S or 4S  or hamsters running in a ladder ring, the amount of HP at the prop to total weight of the boat is what is the basic importance to good hull performance and economy
What a person goes for is dependent on how deep pockets are.
 I retired not long ago.. A new $S was well out of our budget...went to a 20" leg V6 2S 150 johnny.. was looking for a 140 V4 20" leg
 
The other thing may run into issues with up grading is the stern.. I assume the current is 20"  and the availability of 20" 4S or more modern outboards.
 I do not know about this..

The other alternative is  get the stern built up for a 25" leg
 Or go the adjustable bracket way.
 A note here.. very important and will mention now before to late and forget to..
 If building up the transom for longer leg .. say 20" to 25" add 6" not 5" because the angle of the transom (usually 12.5 deg) has to be taken into account.

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Post Options Post Options   Likes (0) Likes(0)   Quote ofthesea Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2019 at 10:54pm
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Do you mean your current old motor is a 1993? When I replaced my 22 year old Evinrude 2S with a new Yamaha 2S same size it seemed that economy was twice as good
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Post Options Post Options   Likes (0) Likes(0)   Quote MacSkipper Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2019 at 12:55am
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Originally posted by Mejiro Mejiro wrote:

Thank you for the replies, I did forget to mention that one cylinder is well down on compression. The motor still starts and idles better than any similar motor I know of and runs well thru the rev range. It's possibly heavier than other tinnys as Alicraft were a jet boat manufacturer and from memory were 6mm hull and 5mmm sides and built strong. Top speed has been 29 knots, I think revs around 5500.
When I've been out its usually 2-3 pob and used for fishing with no heavy loads on board.
There are no visible numbers on the prop without taking it off so it could be under propped, it does perform beautifully in everything except top speed and economy.
It is a stainless prop and from a lot of reading on here the engine height looks right.
The last couple of years have been rough and two weeks ago I bit the bullet and advertised the boat, then I got a break, a small mortgage that reduces my debt repayments by 80% and allows me to upgrade a few things that need it and I think I'd have a lot more confidence  with a later model and more economical outboard.
Steps I can now see that more HP is a good way to go thank you.
68 year old pensioner so economy is much better than speed


it's the boating question that comes up sooner or later to put more money into existing motor or replace?
Though it sounds like you have already decided?   

Has the fuel economy recently taken a dip or always this thirsty?
It sounds like boat was setup correctly?
Dont forget there will be unexpected expenses with a repower - do you have the skills or know someone who can investigate compression loss? As it still runs it may just be one bore and used parts might be available? Just saying sometimes the devil you know....it may be more cost effective to recondition existing how much longer will you be wanting to boat?
Good luck and good on you for asking!
Good fishing trip nothing breaks, great trip catch fish.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2019 at 8:09am
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Dont forget there will be unexpected expenses with a repower

 Is it 1993?

You will find going to a later model johnny or evinrude you will not have to swap out all the looms gauges cables.

 
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Post Options Post Options   Likes (0) Likes(0)   Quote Rozboon Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2019 at 9:33am
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Originally posted by Steps Steps wrote:

Dont forget there will be unexpected expenses with a repower

 Is it 1993?

You will find going to a later model johnny or evinrude you will not have to swap out all the looms gauges cables.

 

If he's looking for economy, nothing that works with his existing gear will do much that's useful.

Johnson's 4-strokes are all actually Suzukis, and if we're talking Evinrude that means Etec which is probably also incompatible.

To be honest, it's a good opportunity to get some modern gauges etc as well.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2019 at 4:33pm
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Difference between better economy and economy and both of those are a matter how deep pockets are..

Even updating to a later model 2000s 2S p0lus better power will make very significant difference to an older early 1990s  2S technology...

 Or .. I should have considered this before... maybe up date the whole rig with a lighter hull...the gear ppl would remain a constant...???

 Again  depth of pockets.
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Post Options Post Options   Likes (0) Likes(0)   Quote puff Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2019 at 6:20pm
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Didn’t he ask about an Optimax? Which is a dfi 2 stroke... it will be substantially better on fuel and economy than a carb 2s, andvery close to a 4 stroke.
Optimax would be all good, just make sure you get it checked out.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2019 at 7:17pm
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Yep...
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Post Options Post Options   Likes (0) Likes(0)   Quote Mejiro Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2019 at 1:57pm
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Thank you again,
1951 for me Steps but 52 was a good year too Tongue
The boat and outboard are both 1993.
4 stroke would be ideal but beyond my budget specially allowing for unexpected costs. I know with an Opti I'd need at least a new battery.
I have a very good outboard mechanic workshop across the road and a mechanical godson who has impressed me with repowers for family and friends who is very keen to help and will be my main fishing buddy up here so that will help save some expense
I have already decided to repower so now it comes down to what I repower with. A 135 Opti sounds good but I had one on a 6.1 Bluefin and to me it was overt powered so from that, a 115 seems like a good option. Depending on what's available when I get the funds a 135/40 conventional 2 stroke may be what I go for.
I'll keep an eye on here and let you know and while I'm here, if any of you ever get the urge to come up for a fish, peace and quiet and about 120 minutes to the Taipa boat ramp, there's enough flat ground on my 2 1/2 acres for a couple of tents and parking for cars/boats and once my boat is sorted you're welcome to come out on that also.

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Post Options Post Options   Likes (0) Likes(0)   Quote MacSkipper Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2019 at 5:18pm
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Originally posted by Mejiro Mejiro wrote:

Thank you again,
1951 for me Steps but 52 was a good year too Tongue
The boat and outboard are both 1993.
4 stroke would be ideal but beyond my budget specially allowing for unexpected costs. I know with an Opti I'd need at least a new battery.
I have a very good outboard mechanic workshop across the road and a mechanical godson who has impressed me with repowers for family and friends who is very keen to help and will be my main fishing buddy up here so that will help save some expense
I have already decided to repower so now it comes down to what I repower with. A 135 Opti sounds good but I had one on a 6.1 Bluefin and to me it was overt powered so from that, a 115 seems like a good option. Depending on what's available when I get the funds a 135/40 conventional 2 stroke may be what I go for.
I'll keep an eye on here and let you know and while I'm here, if any of you ever get the urge to come up for a fish, peace and quiet and about 120 minutes to the Taipa boat ramp, there's enough flat ground on my 2 1/2 acres for a couple of tents and parking for cars/boats and once my boat is sorted you're welcome to come out on that also.

sounds like a good plan just need it all to come together,
Thanks for the invite I bet the fishing is good up there!
Good fishing trip nothing breaks, great trip catch fish.
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Post Options Post Options   Likes (0) Likes(0)   Quote puff Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2019 at 6:30pm
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Put a Yamaha hpdi on your radar too...
I got one and they are awesome....
I’d be keen on a fishing trip too...😃
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 12 Dec 2019 at 8:29am
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A 135 Opti sounds good but I had one on a 6.1 Bluefin and to me it was overt powered so from that,

It would not have been.. it seems that you have not had the opportunity to regular use a well powered or even a little over powered boat.
 The difference in economy thru to hull performance in chop is very dramatically different..total different seemingly hulls.
A 6.1 blue finwould be min powered 175 (maybe a bit higher) well in the 200 hp range

a 115 seems like a good option
 Thats a min for a boat around the 1200kg mark.. like a SN commander 3 guys up 80L tank full

Depending on what's available when I get the funds a 135/40 conventional 2 stroke may be what I go for.
 
That well (nicely) powered for the commander and other similar like 5.5 rayglass

I picked up my V6 150 93hrs full service history winterized every yr, ex taupo engine for 9K
Cost to swap the 115 out new in around $30 to $50.. sealant etc as have my own home workshop engine hoists etc. Sold the 115 for 7K

  We estimate your boat around the 1700/1800 thats about the weight of a buccaneer 635.. That nicely powers is around 200hp  min 175.

The difference between a nicely powered and a min..
A 6m or 6.5 hull will be more uncomfortable and slower and use far more gas working the throttle, in a bit of chop and even more so if chops up than a smaller 5.5m well powered

Its not about speed, but if you want to get somewhere quick and dont mind the extra fuel, you can do so... at a fast cruise.

 Havnt been up there for near on 40yrs
And yeah I know the fishing is good up that way...

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Post Options Post Options   Likes (0) Likes(0)   Quote Rozboon Quote  Post ReplyReply Direct Link To This Post Posted: 12 Dec 2019 at 10:26am
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Originally posted by Steps Steps wrote:

A 135 Opti sounds good but I had one on a 6.1 Bluefin and to me it was overt powered so from that,

It would not have been.. it seems that you have not had the opportunity to regular use a well powered or even a little over powered boat.
 The difference in economy thru to hull performance in chop is very dramatically different..total different seemingly hulls.
A 6.1 blue finwould be min powered 175 (maybe a bit higher) well in the 200 hp range

a 115 seems like a good option
 Thats a min for a boat around the 1200kg mark.. like a SN commander 3 guys up 80L tank full

Depending on what's available when I get the funds a 135/40 conventional 2 stroke may be what I go for.
 
That well (nicely) powered for the commander and other similar like 5.5 rayglass

I picked up my V6 150 93hrs full service history winterized every yr, ex taupo engine for 9K
Cost to swap the 115 out new in around $30 to $50.. sealant etc as have my own home workshop engine hoists etc. Sold the 115 for 7K

  We estimate your boat around the 1700/1800 thats about the weight of a buccaneer 635.. That nicely powers is around 200hp  min 175.

The difference between a nicely powered and a min..
A 6m or 6.5 hull will be more uncomfortable and slower and use far more gas working the throttle, in a bit of chop and even more so if chops up than a smaller 5.5m well powered

Its not about speed, but if you want to get somewhere quick and dont mind the extra fuel, you can do so... at a fast cruise.

 Havnt been up there for near on 40yrs
And yeah I know the fishing is good up that way...


Come on man it's a 5.8m alloy boat. 200hp... he's not trying to pull 3 waterskiers with it.

An AMF ProSport 580, which is arguably the most over-built 5.8m alloy boat on the market (6mm hull plate), has a manufacturer max of 140hp, hull weight 750kg (which is massively heavier than the equivalent, say, Surtees 575 at 585kg, manufacturer max is 115hp).

So something in your maths is way, way, way out. Also on the Bluefin... a review ran one with a Mercury 90hp 2-stroke and got 38mph @ 5500rpm. Tow weight (BMT!!!!) is 1160kg and manufacturer specified maximum is 125hp.

May want to go check your working on this one.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 12 Dec 2019 at 11:04am
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Going from info available the 93 Alicraft Centurion is a very heavy boat around 1700kg gross weight on the water...
 And it is this boat/ gross weight we are re powering to..

That up there with a well powered buccaneer 635.. which happens to have a 6m hull Well powered thats a 200 hp at the prop.
 As you will know ... or should know the co efficient constants of modern plaining hull trailer boats is limited... nah not going explain all that... to get into anther 5hit fight with you again.. you will know all that stuff right?

 And if comparing to model numbers rather than actual hull lengths the 635 is actually 6m  and the surtees model number similar..
 The blue fin I have taken a stab , guesstimate to...If Im wrong on gross weight Im wrong.
 The rayglass 5.5m hull (not model number) the commander 5.5 both weight damn near the same on the water 3 guys up.
A 1200/1300 kg trailer planing boat for general use , regardless of manufacture claims etc, WILL have a best hull performance at 135/140 hp
 And a 1750/ 1850  at 200 hp 175 min

you are a maths man you go do the sums..WITH trailer boat constants (surface area) adjusted accordingly.

 Bottom line its about the gross weight of the Centurion that the subject is about...

OH if you at best can only go off sales / marketing reviews and numbers ... made on min loaded boats , max 2 fit ppl, then proped to the max for that weight...
crunch some numbers talk to actual ppl who do these reviews .. off the record...
You to will end up like so many others .. "why doesn't my boat go like it did on the demo...can we change the prop to make it do so? 
 Answer is its a power thing not a propping thing
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 12 Dec 2019 at 11:14am
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Best rule of thumb I've seen is 25pounds per hp ideal, 40pounds max. 

That's between 11.5kg and 18kg per peak hp on the boat. More than 20kg per hp may not plane. That's for a semi vee type fishing hull not a really deep deadrise, or for skiing. 

This rule of thumb works for a great variety of boats. Weight matters more than hull shape for most normal boats. 

Assuming a dry hull weight of 750kg, 100kg for fuel and gear, 180kg for motor,oil, prop, and 4 people at 80kg each gets to 1350kg. If the boat was lighter, and only 1 up the weight would be about 1000kg. 

I'd rather assume 1350kg closer to the mark (OP states a 6mm hull.)

1350/18=75hp (you'd need to prop it very carefully to plane at all, or put the family on a diet.)
1350/11=122hp. 

115hp should do it, with the right prop, and if I were thinking bar crossings, skiing or unsure of the numbers I'd go for more.

Since you are buying a motor you don't want to play games with min power. If you already had a good 75/90hp I'd fit it up and see how it goes. 


If you put your trailer on a scale at a landfill, we might get a better idea of the actual weight.  Or you can measure the weight of your boat on the trailer with a bathroom scale and some physics.


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