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Advice needed on repowering a 5.8 tinny

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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 11:39am
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Originally posted by Steps Steps wrote:

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..

Can you provide some more detail about how you derived this weight?

OneWayTraffic has done the rough numbers for a 750kg hull, being the heaviest alloy 5.8m I know of, and likely a good match of the stated 6mm hull, and come out to 1350kg on the water.

So unless he's carrying 350kg of ballast, where do you get the extra from?

Genuinely interested to know how you reached 1700kg for weight.
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Post Options Post Options   Likes (0) Likes(0)   Quote Mejiro Quote  Post ReplyReply Direct Link To This Post Posted: 12 Dec 2019 at 11:45am
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I'm a bit confused on this, Steps and Tagit are my kind of go to posts when I'm looking for info in various threads but in this case from towing, launching and retreiving the Ali my estimate is lighter than the Bluefin..... and the Bluefin is top rated at 125 hp and I had discussions by email with Sportscraft boats before buying the 135 because it was over rated for the boat.
Steps your original guesstimate was up to 1350kgs and last post 1750 to 1850. I'm wondering if you've since looked at specs for a different breed of Alicraft. I have a mate with a Commander and it's a massively heavy boat compared to my Ali.
Puff, I will include the Yammy HPDI in my options and if you want to bring your boat up and camp out here you're welcome, just PM me. Same for anyone here, I appreciate all replies. The bay does get hit hard at  times by comms so the fishing isn't always what I expected when I moved up here but having said that, I haven't been getting out enough and there are still places like Rangiputa I haven't tried which are on my list once the boat is sorted

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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 2:44pm
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Steps and Tagit are my kind of go to posts when I'm looking for info in various threads

OK when I 1st started out on this.. Tagit, A retired boat builder / designer in the Sth Island and a guy in Wisconsin, Greer (must have text book) Crouch are all the go tos.. Dave/ Tagit , as I have mentioned in old posts was the one who pointed me in the right direction (not told me.. let me work it out for myself and confirmed when I thought had it right when questions arose.
 Then started building my Data bases
 The big anomalies arise when going between 'moored' boats and trailer boats.
 Displacement and semi displacement..are totally different kettle of fish..more hull length than weight thu semi displacement is getting into planing hulls trolling

Steps your original guesstimate was up to 1350kgs and last post 1750 to 1850.
 My mistake then... I have several 'projects on the go.. powering propping , at once and crossed my wires then.
115 on a 1250kg commander ( and contrary to popular belief SN are not heavy built even on modern standards...their construction was well ahead of their time unlike many other hand laid designs... is a min power... 140 good
 If you come in at the 1350kgs ...135 will be between min and best...a new 115 will make it seem like a new boat...
but till you regularly trip in a well powered, the hull will not be performing as best it could when chop , wakes etc come up

 OneWayTraffic. Thumbs Up
 yes your are near on the mark back there X weight per HP.
Have done a bit on that concept in comparison to Crouche etc...I found it is a ok quick calculation method for smaller boats.. under 900kg.. get above that still a good ball park if round up  NOT down due to the semi logarithmic comparison between weight and hp as boats get heavier.

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.
 Round that up you have 135/140hp.. a good mid range between well powered and the 115 min


115hp should do it, with the right prop
 No matter what, to get results, the hull and engine are propped to the final use of the boat with correct slip numbers ,
eg for a tow boat.. say ski you will go lower pitch slightly less diameter than a general use fishing... if lower mid range or min power
 If you are well powered.. will not matter..
A race boat totaly different again, from prop basic design slips , pitches.
 Propping should always be correct to the end use .. well as correct as stock off the shelf props will allow.




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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 2:50pm
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Now if you want to weigh your boat.. on the trailer dont have to take the boat off, and if duel axle.. remove 1 set of wheels..
A tape measure
 A bit of 4x2
 an accurate set of bathroom scales

 keep the engine tilt as low was possible, but high enough to just clear the ground..

 measure all weights in lbs

 measure All distances in inches

1/Put the sales under the hitch, a length timber long enough to JUST lift the hitch and take the weight on the scales...  with trailer level or slightly low at the front.

record the weight  call it   w1

2/ slide the boat back on the trailer about 12" to 20"

 measure the distance accruately and record it  d1.. the accuracy of this measurement is very critical to the end result

 d1

3/ now with hitch same height record the weight as before You may not be able to move the boat a full 20" as the balance on the trailer may put the hitch up in the air....if so just move back till just have enough weight to measure at the hitch....OR put a known weight on the bow deck.. say a 40L water container full right at the start , before taking 1st weight (W1)

OR simply move a lot of stowed gear equipment well forward.

Measure this weight    w2

4/ now measure the distance from the center  where the bit of wood its on the scales to the center of the axle.

d2

 Boat weight = d2(w1-w2)/d1

If put an extra weight on the bow of the boat, now subtract that weight.

 Now seems strange.. but if you put out all the formulas etc to do the calculations, then simply them.. a whole heap of factors and constants cancel themselves out resulting in the simple formula above.
Basic high school physics and algebra.


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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 11:35pm
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Like the Maths that's how I do it too. Crouches is good as well if you add a safety margin for power.

Here's a link for you to pursue at your leisure Mejiro:


This is unbiased data. Have a look at boats in the size/weight range that you are after. Probably the best indicator of how happy you will be with the motor is the 0-40km/h time. That indicates holeshot, feeling of excess power at the lower speeds when you need to stay on plane in rough seas etc.

They also have the same boat with different motors which is cool. Suzuki marine have the same on their site. Go to the Aussie one. 

5.6m boat little lighter than yours? 115hp.
https://www.yamaha-motor.co.nz/-/media/products/marine/outboard/power-reports/f115/bar-crusher-560c-f115.ashx?la=en

And the same with a 150hp.
https://www.yamaha-motor.co.nz/-/media/products/marine/outboard/power-reports/f150/bar-crusher-560c-f150.ashx?la=en

I personally wouldn't spend a lot extra for the 150hp on that hull. But I certainly wouldn't discourage you if you were of a mind too either. 



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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 13 Dec 2019 at 8:55am
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Crouches is good as well if you add a safety margin for power.

No.
 Crouche is the world std for naval archetics planing hulls.
 And when applied CORRECTLY scary accurate.
 Then combine that up with a prop slip calculator also become uncanny accurate.
 
The issue where ppl go wrong with it is the hull constant.
This is a problem for wider beam  (non trailer boats) as establish a particular constant one must have built a data base of different hulls... And with increased speed so will the constant increase marginally due hull friction area.
 The other issue is applying a constant to trailer boat.. which have limit on bean due to legal restriction on towing on the road.
This results in a very very different set of constants to what Crouche, (and many web calculators) suggest.
 We must look at history to see why.. Crouche was in the 1920s/ 30s.. plainling hulls that we tow around now where not common trailer ed boats with wider beams

One quickly realises once establishing a data base of trailer boats is that the constant is very similar regardless of hull design. Th reason for this is if you take the rear 1/3 of a hull they maybe slightly rounds , or Ved with a keel, but essentially are a near flat surface with little variation in contact area at cruise speeds.

If anyone is serious in following up  powering planning hulls, and propping  get a copy of  Dave Gerr  Propeller Handbook.

Sitting down working thru formula calculations was becoming a mission for me.. So I went the next step.. I tought myself basic Java coding and put into html rather than Excel (the latter can not give all the versatility of options I wanted)  How to make a android app.
 The HTML works nice on computers
 Android works (even better) on Android phones and tabs.


 As you will notice I made it in mph.. just goes back to my old school hot rod/ classic car 'heritage'
And included a converter
 Like everything , it is accuracy of data .. and correct data that is very important.
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Post Options Post Options   Likes (0) Likes(0)   Quote Mejiro Quote  Post ReplyReply Direct Link To This Post Posted: 13 Dec 2019 at 11:57am
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Thanks again for the help.
My brain works on the KISS principle and algebra is an alien language to me, not too sure about physics either.
However....
looking at the Yammy 115 on the Barcrusher 560 it's showing an average top speed about 2 km/h higher than what I get out of my old tech 115 Evinrude running on 2 1/2 cylinders which, to me, indicates that my Ali is lighter than the Barcrusher and should get good performance from a 115 Opti so I'm inclined towards 115/135 Opti or possibly a Yammy HPDI depending on what's available and checks out.
Does that sound right?
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Post Options Post Options   Likes (0) Likes(0)   Quote Mejiro Quote  Post ReplyReply Direct Link To This Post Posted: 13 Dec 2019 at 11:58am
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oh and speeds on my boats are always from GPS reading
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 13 Dec 2019 at 12:36pm
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If you were happy with 115hp before you probably will be now. I'd step up to the 135 if the weight/$$ penalty isn't much. 

Steps by safety margin I meant by not allowing power for acceleration/rough weather/excess load/getting up on the plane/not running full throttle everywhere etc. Crouch gives power to maintain speed for a given load and hull factor. The excess produced by an outboard over that is your F=ma. 

e.g. if I needed 50hp to cruise at 25knots, and never wanted to go faster, I'd still be silly to put a 50hp on. Better to run a 75hp at 2/3 power IMO.




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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 13 Dec 2019 at 7:32pm
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If you were happy with 115hp before you probably will be now. I'd step up to the 135 if the weight/$$ penalty isn't much.
 Thumbs Up
Yep..another thing going to well or very well powered....
 It lack of noise.. no not as a good modern 4s .. a way off still.
Since the engine doesnt work as hard constant up hill drive (which in effect what the engine is doing)  you dont have to raise your voice any where near what you used to do Wink

When you approach a wake at a reasonable speed, you dont need to move the throttle (unless a real big wake)  line up tap the trim down a couple times and go over.. turn back to the direction intended and couple taps up..

I very well powered/ little over powered, you ask if every one has a hand hold AND turn and check... move throttle a little forward, boat moves off gently climbs over the bow wave, then as flattens out .. tap the trim up till just before cavitaion and you are at a nice comfortable cruise.
And depending on conditions ,  and mood, you have the option to travel much faster comfortably, in conditions you could not have before.

In my book, and what I have personally found is each of these make a marked difference to the comfort and mood of the day .....which to me is what it is all about.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 13 Dec 2019 at 9:03pm
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Steps by safety margin I meant by not allowing power for acceleration/rough weather/excess load/getting up on the plane/not running full throttle everywhere etc.
Thumbs Up

 Crouch gives power to maintain speed for a given load and hull factor. The excess produced by an outboard over that is your F=ma.


Yes??
It is taking that min concept/ formula and using it to then be able to not  find out what is, but to predict what will.

If you are bored stop here..
 If curious read on and I hope I explain it ok

Crouches formula is as basic as Newtons basic laws.
"A given force will move a given weight at a given speed."
 All crouche does is added on factors such as friction

The engine manufactures create a engine that will give a power ratio curve suitable for a fixed gear ratio with in effect a variable slip clutch as load change (slip on prop) .
 They then design this curve so that there is a max rpm range that must be met under all loads of the hull.. ie light top of the range..heavy bottom.
Then they then need to gear this down a bit, so throw a reduction box  at the behind the prop.

Going back to Newtons laws.. in case you dont know this is the guy who got interrupted under a apple tree..  one fell on his head..presto gravity.Confused
 if a given force on a given weight (mass) moves it at a given speed... then the opposite will apply
If we know the force and the speed we can easy work out the weight
So in the app If we know the HP at the prop, the max speep on flat water at best trim for THAT speed.. we can work out the weight of the boat...
  Going full circle now...

 Crouch gives power to maintain speed for a given load and hull factor. The excess produced by an outboard over that is your F=ma.

 With the app one can figure out the required hp to move at any speed for that hull (in practice the constant may change  5 to8 pts due to more hull in contact with water and load of other stuff)
You may notice on the app the default numbers
 Both sides are the same boat
left is WoT
 Right is fast cruise.
If I plug the left WoT speed 48.5 into the right..and know the rpms (4700) I can calculate the slip  3.5%

Before someone says something .. this is a prop I was trialing..yes max rpms little low of target (4900)  and slip low.. 4 to 6% target.
My target was 12 to 15% slip at cruise
15 %  top limit slip for slow cruise to see what speed will do at say 3800 it comes out at 34.5 mph
 And if was to put 4050 rpms in @ 13% slip / fast cruise.. I get exactly what the boat actually does.
 In fact all the way up from 3000 rpms thru to WoT  what happens on the water is what happens by plugging numbers in within 0.5% with that prop

So we have different manufactures with different curves and different gear ratios.. but at the end of the day manufactures ALL make so if you hit WoT with boat  light as possible you will hit close to the max rpms of their range.
And load the boat up you will close to hit the bottom..
Regardless of if you have a 90 pushing a 7m on the plain or a 4m.





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Post Options Post Options   Likes (0) Likes(0)   Quote MacSkipper Quote  Post ReplyReply Direct Link To This Post Posted: 13 Dec 2019 at 11:46pm
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Originally posted by Steps Steps wrote:

If you were happy with 115hp before you probably will be now. I'd step up to the 135 if the weight/$$ penalty isn't much.
 Thumbs Up
Yep..another thing going to well or very well powered....
 It lack of noise.. no not as a good modern 4s .. a way off still.
Since the engine doesnt work as hard constant up hill drive (which in effect what the engine is doing)  you dont have to raise your voice any where near what you used to do Wink

When you approach a wake at a reasonable speed, you dont need to move the throttle (unless a real big wake)  line up tap the trim down a couple times and go over.. turn back to the direction intended and couple taps up..

I very well powered/ little over powered, you ask if every one has a hand hold AND turn and check... move throttle a little forward, boat moves off gently climbs over the bow wave, then as flattens out .. tap the trim up till just before cavitaion and you are at a nice comfortable cruise.
And depending on conditions ,  and mood, you have the option to travel much faster comfortably, in conditions you could not have before.

In my book, and what I have personally found is each of these make a marked difference to the comfort and mood of the day .....which to me is what it is all about.
good summary of well powered boat and it's true!
Good fishing trip nothing breaks, great trip catch fish.
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Post Options Post Options   Likes (1) Likes(1)   Quote Mejiro Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jan 2020 at 10:49pm
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I've gone for the 115 Opti and it's fitted and the boat painted to match. Waiting for crew now to try it out
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Post Options Post Options   Likes (1) Likes(1)   Quote Mejiro Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jan 2020 at 10:57pm
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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 2020 at 8:51am
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I only just saw this thread. Looks like a good tidy rig Mejiro. She will perform well


Best gurnard fisherman in my street
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New paint certainly changes the look impressively.
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Post Options Post Options   Likes (0) Likes(0)   Quote Mejiro Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 2020 at 11:45am
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Thanks Steps and Smudge, next will be a hatch for the cabin top to make anchoring easier and safer then a bimini which I'm still designing and have to save for both. The plan is to take her out this weekend, probably fishing going out from Houhora.

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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 16 hours 29 minutes ago at 8:08am
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Bimmini design..
 I put stood in quite a few boats and put a lot of thought into ours...
Have a similar windscreen set up as u.
I went for the clears from screen to canvas top..all round.
A  narrow cnr zipped section each side.
A wider to just past the middle on the crew side...
A narrower for in front of the helm..then another cnr
All can be unzipped independently or together.

The distance back for side clears was measured so could fish 2 ppl  and if fishing 3 , the side clears  could be folded (not tight) inside themselves for more gunnel room... and the rear brace rope removed.
How far back the bimini roof and side clears goes is a bit of juggle for between casting, keep out of wind rain and sun ..a couple inches makes a huge different to all...

I placed the holders, marked heights, distances with a felt tip when actually out using the boat on several trips.... and pretty well nailed them

Modifications:

I did modify the frame by adding another stand up frame and horozonal brace at the top of the side clears.. This was so the whole bimmini remained stable if towing and traveling fast at sea. The horizontal brace also became the mount for strip deck lights
Originally the front clears where meant to roll back over the top and be held in place with velcro.. rubbish idea.
They clip to the top of the windscreen with the hole clip thingies...I now have then attach on the inside frame with straps  with the hole clips attached..and can be moved.
 This enables opening , say the helm window clear, back inside and have the opening adjustable by how far the zips go down and length of the straps.
I can also use the rear vertical frames, with 2x sided velcro to hole my longer SB rods.
And just so can clear the bimini are 2 holders on the gunnels can put 2 rods for quick access, and just clears the top rear of the bimini.
 I will see if have any pics suitable uploaded.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 16 hours 22 minutes ago at 8:15am
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