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Motors again - 70-90hp

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    Posted: 04 Jul 2018 at 3:07pm
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Im thinking about getting a extreme 500. That's an alloy boat, length overall 5.0m. Relatively heavy as uses 5mm/4mm aluminium.
What motor should I get?
I havent been able to  get a ride on a 500, but have tried a Extreme 540 - that's about 10% heavier. With a Yamaha F70, it went max speed 30knots at 5800rpm - in fairly calm, 3 POB. . I would normally have 1 POB and get a higher top speed, that would do me. 
So Im thinking 70hp would be enough. For local servicing availability I would like Yamaha or Mercury - Yamaha F70 vs Mercury 4str 80 or 90.
The Yammie is far lighter at 119kg, with a tiny 1L capacity; so presumably its highly tuned to get that HP. BUT - the Yammie F70 howled like a tortured banshee, I thought it was a two stroke at first. How come a 4 stroke makes that awful high pitched noise? Is that normal - how do others regard that? Im not used to the high pitch, coming from a nice growly Honda.
The bigger Yamaha F90 weighs 162kg, about the same as the Mercuries. Question - does the Yammie 90 sound the same high pitch as the 70?
 
The Mercuries are both 165kg, 43 kg heavier than the Yammie. Wow that's a lot more weight! Capacity is 2.1L, over twice the Yammie. Presumably the Mercs have much more Torque at lower revs. I haven't heard the Mercs, but obviously need to.
 
Im in a dilemma. Take the Yamaha 70 for the lighter weight (but awful howl), or the Yamaha 90 or Merc 90 for quieter sound, but higher weight?
 
What should I do?
  
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Post Options Post Options   Likes (0) Likes(0)   Quote SpearPoint Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jul 2018 at 3:35pm
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The F70 shares the same block with the F50 and F60. The Merc 80 or 90 share the same block with the 100 & 115. I don't think you can really compare them as they will be miles apart in the torque department due to there being 1.1trs difference of displacement between them.

If the boat can take the weight I'd take the Merc not just for the extra HP but for the big increase in torque.

I'm pretty indifferent about noise but that's just me.

What was the difference in price by the way?
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Post Options Post Options   Likes (0) Likes(0)   Quote MikeAqua Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jul 2018 at 4:40pm
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The F70 makes that noise because it's a small block working hard.

I don't think 140HP is enough for that boat.  I also don't think two F70s are enough displacement.   I'd go for 90s.  Fit trim tabs if the stern squats a bit too much.
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Post Options Post Options   Likes (0) Likes(0)   Quote Schampy Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jul 2018 at 5:22pm
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Have you looked at the suzuki 70/90?  They are about 145 kg I think but a much larger block than the yam. (1.8 if I can remember correctly.... its been a while)   Had The 70 suzi on my previous boat..5m center consol and it was an absolute peach of an engine. Compared to the 70 yam I had on surt 485 before that was like night and day.
Suzuki has way more torque... meaning much more relaxed cruising speed and better fuel economy. Loved it.
The 70 Yam sorta suited my surtee 485 better in the sense that the hulls are not very buoyant in the arse. Combined with 2 full tote tanks at the back..... Any thing heavier than a 125 kg engine would have the boat dragging its bum like a dog with worms.  Seemed to have to thrash the engine pretty hard to cover distance though. But with The 5m cc (much heavier than surt.. Ply/glass) The suzuki with bigger displacement and  and superior torque was much better suited to heavier hull. Even though there was a weight penalty....  It didnt phase boat.... she would just lop along at 4200 rpm doing 30 mph all day.
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Post Options Post Options   Likes (0) Likes(0)   Quote Joker Quote  Post ReplyReply Direct Link To This Post Posted: 05 Jul 2018 at 1:34pm
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I've got the Merc 90hp on an alloy 560 C/C and its a perfect fit with lots of torque, quiet, reliable and economical. I have the Command thrust model which is supposed to help it plane at a lower speed - seems to work. 
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Post Options Post Options   Likes (0) Likes(0)   Quote RC1 Quote  Post ReplyReply Direct Link To This Post Posted: 05 Jul 2018 at 6:56pm
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F70 is too small for that hull. You would be best to go for the F75/90 Yamaha. The 75 is a de tuned 90hp and goes really well. If the budget allows go for the F90 Yamaha. These new Yamahas are nice engines.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jul 2018 at 8:36am
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Motor weights will have very little or any effect on a boat that size.
ball park a 5.5m boat total gross weight on the water 1100/ 1200 kg well powered is around 140 hp... min well powered is around 115hp

 What do you think the total gross weight will be? ppl fuel gear everything?
 Go for at least the manufacture max hp rating if want the best riding, chop performance from the boat.
 Anything under that is a marketing thing to sell the biggest boat with the least amount of hp to ppl on tight budget.
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Post Options Post Options   Likes (0) Likes(0)   Quote letsgetem Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jul 2018 at 8:54am
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Its a 5m boat overall length.
 
On the water weight estimate is 950kg with 2 POB.
 
I agree with the idea that dealers like to market a boat with minimal power, to make the price lower. That's exactly what a dealer did, when I went on a Extreme 540 - reckoned don't need any more than 70hp, and any more would be too much weight for it. When I said but the maker recommends 70-115 - there was a deafening silence.
 
I am generally against a motor being highly tuned to get higher power, as it should be less reliable and durable.
 
Good advice so far, keep em coming. 
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jul 2018 at 9:48am
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Rem top speed is an unrequired side 'symtom' of being able to have good reserve power at normal cruise speeds and hitting a chop/ wake stalling etc...
 Also ball park for these trailer 4.6m thru around 6m min well powered around 42mph and well powered high 40s around 47/48 mph  and propped at full normal load to middle or smidgen below the manufactures max rpm range..

So on 1050kg 
 WoT for 42 mph is about 100hp
 Propped well.. pitch and good slip (grip) cruise around 4000 rpms around 32 mph
 for  47 mph 118 hp
 cruise around 3850 rpm around 37 mph on flat water.
 If go larger engine and cruise at smaller engine speed (32mph) you will have around an extra 7 to 12% better economy... 
cruise at the faster speed and around 5 to 7% more fuel than the smaller engine at slower cruise speed. 

 Get in heavey chop on the smaller engine you will be working the throttle a little to stay on plain, and crew thru more gas..
 On the larger engine you will not have to work the throttle.. set and go..travel more comfortable at a 2 to 3 mph faster and the fuel consumption drops dramatically to well below the normal slow cruise.
 Put a little more weight in , say extra guy and gear , say 90 to 110 kg, the smaller engine will cruise couple mph slower, use more fuel, more 'pig' in chop.
 larger engine, hardly notice the diff, 50 to 75 rpms to maintain same cruise speed, stuff all change in consumption. And heavy chop, no change other than the extra weight will improve ride a little more.

Over power say plus another 10 hp and all the benefits of the larger engine get better....just get in the habit that every time you take off, "all holding on?"
 Cause you will be just in gear, trimmed right down, ask, move throttle, a little forward, let it go, as goes over the bow wave trim up and will be at 3800/ 4000 cruise speed...and have not touched the throttle.
 If they are not holding on they will be on the floor or over the back... little over stated but certainly applies.

I have a lot ppl contact me .. "my boat is not going as well as the original (sales) sea trial... (not fully loaded) can you see if propped right?"
8 times out of 10 it is
1 time, a sight change makes an improvement but not significant to warrant the capital cost of a new prop
Other time is baddly propped , but still cant fix being min to under powered.

Also of interest, several  hull manufacturers, often with a small charge, will increase the load tags for a little more hp for insurance purposes...I assume therefore there tends to be a substantial margin in some hulls.
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Post Options Post Options   Likes (0) Likes(0)   Quote letsgetem Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jul 2018 at 10:09am
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I estimated the boat weight wrong - included the trailer - more like 950kg I think, but don't need to assess it again - I can allow for that. Thanks.
 
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jul 2018 at 11:00am
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I have been taking note of tow weight and on water weight.. just as a side interest comparison.. and not thinking as a good example...
 BUT in general 4m to around 6m the tow weight plus people is a pretty damn close.. little surprising...
 Also be careful what the advertised tow weight is.. Some its the net boat trailer weight, 
others it is  trailer net boat plus a motor,
 and others boat trailer, motor fuel and basic gear(batteries)
 Its a bit like advertised hull length.. bow to transom or bow to transom plus any platforms out the back, or even in some cases includes a bow sprit.

 I dont need to get out calculator etc .. I built a app, punch numbers in , any variable and comes up with the calculations... same with the propping.
So At 950kg 47 mph is around 106hp
at 42 mph around 92hp

If anyone has the same boat and can post their hp and speed @ WoT  roughly number ppl fuel  we can get it closer.

 PS modern outboards espec 4S  tend to under rate their HP about 5 to 10%
 4 S tends to have a flatter rpm /hp curve below 4000 rpms, having more hp means they pull up as well as a 2S and similar reserve power Wink
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Post Options Post Options   Likes (0) Likes(0)   Quote letsgetem Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jul 2018 at 11:09am
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I take that point - tow weights given by the manufacturer are no doubt variable, as they probably want to make it appear lighter so people think its easier to tow.
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Post Options Post Options   Likes (0) Likes(0)   Quote rowboat bob Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jul 2018 at 9:16pm
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I'd go for the Merc 90.  I've recently put the Merc 115 Pro XS from the same range,  on my 5.5 m glass boat and it's awesome. What sold me on the Mercs were that although being the latest technology , they are also the simplest and although having the largest displacement at 2.1L they are also the lightest in their class. The designers of this range of Merc engines have gone for "lazy horsepower" which basically means the motors are not working too hard. This approach should be good for reliability and longevity, while the simple,  single cam with factory set valve timing should keep servicing costs down.
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Post Options Post Options   Likes (0) Likes(0)   Quote letsgetem Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jul 2018 at 5:10pm
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A question I need to answer, - will a 160kg motor (4stroke 80-90hp) be too heavy for this boat. How do I decide that?
The maker's representative seems a bit unsure - on their website says 70-90 is ok, but later says 50-70 is recommended.. And so far they havent come up with any 500 that's got a 160kg motor for me to enquire about. Im wary about it - as Ive heard of boats that are too heavy in the stern.
Its beam 2.1m. Underfloor fuel tank 80L, that's weight near the stern. Anyone got any ideas? Anyone got a 80-90hp 4 stroke weighing about 160kg on a 2.1m wide boat?  
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jul 2018 at 6:03pm
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will a 160kg motor (4stroke 80-90hp) be too heavy for this boat. How do I decide that?
 will be fine.. thats about the weight of older johnnies evinrudes , merc s etc that where common on boats that size.
 From memory the weight of 70 to 115 johnnie is about 160/170kg and the 150 V6 200kg
 The extra 25 off kg on the back of the 5.5 commander made nil difference....even with a couple 10L buckets of muscle added on top sitting back there..
Think about it like this.. you decide to take the grandchildren out and they both decide to sit in the stern...make that much difference? nah

 If you are talking 4 or 4.5 m then you will start to consider that... maybe.. but then you will be well over powering that boat.. becomes a competition speed boat right? 
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Post Options Post Options   Likes (0) Likes(0)   Quote letsgetem Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jul 2018 at 9:15am
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Two dealers of extreme boats, have said they think 160kg motor on an Extreme 500 is too heavy. Because, the length of the hull is a factor, and its not long enough to counterbalance that weight on the back - resulting in being too hard to get it planing, ie have to use more power and speed than desirable.
The Extreme 500 is relatively heavy for its length - because it uses thicker aluminium than other makes, 5mm hull and 4mm superstructure. The higher weight, needs higher power to push it adequately. To get the higher power, motors are heavier. The only motor that appears to be enough power, is the Yamaha 70. Its remarkably light weight at 119kg, compared to others 70-115 around 160kg. But, the Yammie 70 makes a high pitched noise, that is not acceptable to me. And it needs to run at relatively high revs to get the power, which increases the noise level.
I cant see any way around that, so Ive scrubbed the extreme 500 from my wish list.
That's ok to me, its also very expensive.
Something else will take my fancy, theres no hurry, and I might not change my existing boat (Stabi 1530).
As I said to the dealer (sales bloke), previous boats have all had something that pisses me off, and the next boat (if any) has to be just right.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jul 2018 at 10:06am
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OK..
 understand the comments  MAY have a merit.. in saying that is holeshot that important?.

 Is the hull actually 5m bow to stern on the 500, or does that include the rear duck boards?

resulting in being too hard to get it planing, ie have to use more power and speed than desirable.
 Both of these are rubbish..
 The hull will have a constant, same as any other trailer boat.. between 195 (cruise) and 205 (WoT) its to do with the area incontact with the water on the plain at a given speed.
 Any power is about moving a given weight at a given speed over a given distance. If you where underpowered and dumped a big heavy engine on ... yes then what they say would eventuate...
 Trouble is to source a real heavy motor with too little power doesnt exist anymore.

With reasonable power , fully trimmed down should still not be an issue.
 The important part for general  use is, I believe good cruise speed, good power for chop (dont have to play the throttle to stay on plain).
 you could dump a heavy V6 on the back, nose will point skywards intial hole shot for a second or to, bow will go over the bow wave, hull will flatten right out you will have thumb on the trim, trim up real quick and be skimming the water surface nps
 
in fairly calm, 3 POB.
 How many do you generally have on board?
We find 5.5m fishes 3 ok , 4 bit cramped....
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Post Options Post Options   Likes (0) Likes(0)   Quote Joker Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jul 2018 at 6:50pm
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Forget the Yami 70 - its really only a 60hp that gets the extra hp at WOT but doesn't have the mid range thrust that you need. A 5m boat with a 2.1 beam can handle the weight at the back but to be sure I would put a 4 blade prop on it to get the extra lift or go the CT model of the Merc 90.  
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jul 2018 at 9:42am
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Forget the Yami 70 - its really only a 60hp that gets the extra hp at WOT but doesn't have the mid range thrust that you need.

 Thats a damn good point.. 4s by nature dont have as a high power curve down low mid range like 2S...which maybe is why 4S tend to be underrated  5/10% to adverttised WoT hp.
 Get to the max ci of a range and that will be very likely to show up when hit a good chop. 
Since the 4s have started to be marketed its damn hard to find any hp/rpm graphs for them....other than where workshops have done actual sea trial numbers.
 Put those 2 things together and one wonders just how much of the 4S marketing performance propaganda is up front and transparent. 
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Post Options Post Options   Likes (0) Likes(0)   Quote Big -Dave Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jul 2018 at 8:59pm
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4 strokes with variable cam timing/lift, and variable intake manifold lengths, produce better mid range torque.

Everyone is going to have differing ideas on how much power is necessary.
I found my boat with 3 pob, approx 2000kg, gotup and went fine with 130 hp, the only time it was an issue was climbing up the back of a steep swell, something i rarely had to do.or, when i had 6 or 8 pob.
A bit of gear stowed up front, helps keep the nose down if your motor is a bit heavy on the back.
Wedging the motor to get it to tuck in more, also helps.
you can't fix an idiot with duct tape, but it does muffle them for a while...
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