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Discussion Starter · #1 ·
How many knots should a 35' boat make under 100% power? Should it be able to reach hull speed with room to spare?
What speed would you expect to motor along at a "cruising" speed of 75-80% of full throttle?
Assume flat water, no current, etc.
The reason I'm asking is that I'm looking at a new boat that size but I'm wondering if the engine is under sized.
Thanks.
 

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To some extent this is like asking how many horsepower an automobile should have - it is all a matter of opinion.

Personally I would want 6 knots at "cruising" speed, and to easily reach hull speed in reasonably calm water at a minimum. It think it is nice to have a little extra in reserve to fight a chop if needed.

Assuming your 35' boat is a typical cruiser (i.e. not a light race boat or a super heavy cruiser) I would expect about 25-30 hp.
 

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There are many factors that get involved here, engine horsepower is but a tiny one.

If the wrong prop is on the boat (or if an adjustable feathering prop is misadjusted) then you could have trouble making hull speed. A Yankee 30 that I sail on has the wrong pitch on it's Max-Prop and we have to run it's Beta 16 at 2800rpm to make 4 knots, the boat should be making about 6 knots at that engine speed. Imagine if a sporty car was missing the top two gears in a 5sp transmission, it would be the same thing. The car might be designed to drive 100mph nicely, but would be limited to 60-70mph by the transmission being stuck in 3rd gear, not the engine.

Sailboats tend to dig in the stern under power, leaving a lot of turbulence. On my boat I can go about half a knot faster under sail than under power for that reason alone.

A dirty bottom can really slow the boat down and prevent it from reaching hull speed, even if the engine is the correct size.
 

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Assuming that this boat is a displacement hull, you should be able to achieve "hull speed" somewhere between 65% and 75% of max throttle, in calm water with no current.

"hull speed" is defined here: Hull speed

It has to do with waterline length, not LOA. So, you can't simply say "a 35 foot boat, should drive to X knots". It's not that simple.

Do the math, figure out your theoretical maximum hull speed, and compare it to what this 21hp engine is giving you. The extra 25% available throttle margin, is helpful for powering into wind or chop, should the need arise.
 

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On a friend's late model Jeanneau (36/37?) with Yanmar 3GM30 (27hp) and folder we cruise at 6.5-7 knots, same engine in my Cat320 with 3 blade I cruise at 5.5-6. Hull speed for the J is about 7.5 mine's around 6.5 I think.
I understand your concern, builders used to empathize that it was an "auxiliary" engine and used smaller engines, it was just something to get you on and off the dock. Today's boats are built with the reality that you have to go to work on Monday and there might be no wind Sunday to get you home.
 

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Hey,

My O'day 35 has the Universal diesel M25 which is rated at 21HP. I have a fixed 3 blade prop. When the prop and hull are clean I can do 7+ kts in flat water at full throttle. I can cruise at 6-6.5 kts at 2200-2400 rpm.

The o'day weighs around 12000 lbs. I don't feel like it is underpowered at all. Perhaps if I had an inefficient folding prop it would be, but with the 3 blade I can motor at good speed all day long (not that I like to).

My C&C 110 has a 29HP saildrive with folding prop. I can't really motor any faster than the O'day. Maybe I can cruise at .5 kts faster but that's about it. I attribute that to the prop difference (and the fact that the C&C hull speed is only .5 kts more than the o'day).

Once you get close to hull speed, you start needing lots and lots more HP to go just a little bit faster.

Barry

How many knots should a 35' boat make under 100% power? Should it be able to reach hull speed with room to spare?
What speed would you expect to motor along at a "cruising" speed of 75-80% of full throttle?
Assume flat water, no current, etc.
The reason I'm asking is that I'm looking at a new boat that size but I'm wondering if the engine is under sized.
Thanks.
 

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Boat displacement is everything. Rule of thumb is about 2 HP per 1000 lbs boat displacement.

We're 16000# with 33 HP and that seems right. We can easily get to hull speed at 80% power.

Our last boat was 8000 lbs with 13 HP and had problems in any seaway.

The rule of thumb seems to fit the data.
 

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Discussion Starter · #10 · (Edited)
The builder has given me data from a sea trial showing maximum speed of about 7.2 kts at WOT and 5.7 kts at 78% WOT.

These numbers worry me.

The prop is a 3 blade fixed and the test appears to be done with some tanks filled, gear, and crew.
 

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Details! You've been asked for a number of different details and so far all you have said is that the numbers you have worry you. You're not going to get any useful help unless you answer the questions you've been asked.
 

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Beneteau 393
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What sort of boat is it? Please tell us because we are crawling in the dark otherwise.

For an old heavy 35 foot LOA with say 28 ft LWL its exceptionaly good. For a modern design almost 35 foot LOA is not so flash.


If you are too scared to put the type of boat on the net for fear of someone buying it from under you just the LWL and displacement.
 

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Discussion Starter · #14 ·
What sort of boat is it? Please tell us because we are crawling in the dark otherwise.

For an old heavy 35 foot LOA with say 28 ft LWL its exceptionaly good. For a modern design almost 35 foot LOA is not so flash.

If you are too scared to put the type of boat on the net for fear of someone buying it from under you just the LWL and displacement.
lwl=31
disp = about 15k loaded.

The builder sent me results of a sea trial... but I'm not sure if they would want me to put it on the web.

The speed at cruising seems very low to me and the speed at WOT seems like it should at least be able to get to hull speed.
 

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It sounds a little underpropped perhaps, but not underpowered. Otherwise there wouldn't be such an increase in speed going from 78% to 100% of engine RPMs. Prop pitch is the cheapest part of the system to change.
 

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For your comparison - we have a 34 Irwin Citation

  • Length 34' 7.5"
  • LWL 27' 4"
  • Displacement 11,500 (13,000 loaded) lbs
  • Yanmar 2QM20
  • RPMs and speed in knots loaded
  • 2600 RPMs 6.9 -7.0 (Full throttle)
  • 2500 RPMs 6.7
  • 2400 RPMs 6.6 - 6.7
  • 2200 RPMs 6.3 - 6.4
  • 2100 RPMs 6.2
  • 2000 RPMs 5.7 - 5.8

Rik
 

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I agree that it's underpowered. We're 16000 lbs with 33 Hp. I think that 30 Hp is more in line.

But if you love the boat, add in extra time to get places. OTOH, I doubt that you'll do 7.2 kts under sail that often, so why worry at all? Seriously, its sailboat. No offense.
 

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21hp is slightly underpowered for a 15k lbs. boat, but not terribly so. Should probably be closer to 30hp. Still, in calm water, 21hp should be able to get the boat to hull speed and it should certainly be adequate for most circumstances.

I would agree with those who say the prop is probably not ideal for this boat. What's more, you need to be sure that you understand the speed numbers that you have. How were they measured? If by the boat's speed log, is it properly calibrated? Most that I've seen are off by at least a little, some by a lot. If by GPS, was current taken into account? That could make a difference of several knots.

Good luck, whatever you decide to do.
 

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Beneteau 393
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Hull speed of a 31 ft lwl is 7.5 and thats theoretical and could be less than that as yours is very heavy displacement.

Your speeds sound fine.
 
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