Beneteau 36CC does not reach hull speed - SailNet Community

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Old 06-27-2012
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Beneteau 36CC does not reach hull speed

Hi all:

I own a 1999 Beneteau 36CC that I purchased new in 2000.

She is equipped with a 27HP Yanmar 3GM30F and is fitted with a 3 Bladed MaxProp.

I am quite certain that the pitch is set correctly because with full throttle, the engine RPMs are 3600.

Anyway, my situation is that last weekend my boat speed was 5.1 knots at 3200 RPM. Calculated hull speed on the boat is 7.48 knots.

There was a 9mph head wind and a .5 knot current against me, but 5.1 knots seems a bit low.

My bottom was cleaned a couple of weeks ago. Also, I was carrying a full load of water and fuel and I do have an all chain rode.

My question to other 36CC owners is, "Does this speed seem normal to you?"

Thanks.
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Old 06-27-2012
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Re: Beneteau 36CC does not reach hull speed

Are you going by a GPS or paddlewheel instrument? If the latter, the problem may be with the instrument.
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Old 06-27-2012
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Re: Beneteau 36CC does not reach hull speed

Hull speed is not the max. speed of the boat but rather the speed of the WAVES which are produced by the bow and stern and which AT calculated 'hull speed' are still fully supporting both the bow and more importantly the stern. Beyond 'hull speed' the produced stern wave will be 'behind' the boats stern and no longer fully supporting the stern ... and the boat begins to 'squat' because the stern is no longer supported by the stern wave.

Other considerations: Running a boat in relatively shallow water will cause the bow and stern waves to become greater in amplitude and with the bow and stern waves 'slower' ... wave 'reflection' off the bottom. It takes more power to attain the same speed in relatively shallow water than when in deeper water. ;-)

To determine speed vs. engine output (rpm) either assay this in FLAT (deep) water with no to little wind blowing ... or less preferably, run several 180į courses and then average the results. Just plot several speeds vs. engine rpm ... and where the line you plotted starts to rapidly change direction ... is your 'functional' hull speed.

Prop 'efficiency' has a lot to do with how much 'speed' you get out of a boat and no matter how fast the engine is turning, even if the engine rpm is developing maximum HP from the engine.

A .5 knot current against you, plus a 9 kt headwind can easily drop your speed vs. 'hull speed'. Do several 'assays' in various light wind and zero current conditions ... and then and only then if the expected isnt resulted, .... then search for the cause.

Also be aware that most 'boat instruments' arent 'calibrated' and only give 'approximate' and sometimes 'variable' results. ;-)
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Last edited by RichH; 06-27-2012 at 01:58 PM.
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Re: Beneteau 36CC does not reach hull speed

I am measuring the boats speed via GPS.
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Re: Beneteau 36CC does not reach hull speed

Thanks for the detailed response.

First, water depth was over 75'.

Second, you are saying hull speed is not the max speed of the boat, but the speed of the waves.

At 2800 RPM or greater, my boat does squat with the lower edge of the transom lowering toward the level of the water, but I am nowhere near hull speed when this happens.

Are you suggesting that I am going slow, because I am operating the boat at too high an RPM?

When I operate the boat at 2800 RPM I only get to 4 knots.
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Old 06-27-2012
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Re: Beneteau 36CC does not reach hull speed

I am a naval architect. Webb '82.

Hull speed isn't a real thing. When all hull forms were much the same there was an empirical relationship between waterline length and reasonable speed. Even so, it isn't a specific number. It is only an approximation of a point in the "knee of the curve" that shows the relationship between power and boat speed. Even there it ignores external effects like current and head winds.

5 knots on a 36 foot boat (31 foot waterline?) in an adverse current, headwind, and any sea at all is fine.

Empirical parameters like "hull speed" really aren't used much anymore as more deterministic modeling has become so inexpensively available.
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Old 06-28-2012
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Re: Beneteau 36CC does not reach hull speed

I have the same engine and a 2.21:1 ratio gearbox in a C&C 32. I believe my hull speed is around 6.5 knots.
At about 12,000 lbs. fully loaded and with a 16"x14" fixed 3 blade prop at 2800 rpm my boat will cruise at 7 gps knots in any reasonable conditions.
It sounds like your boat may be under propped or under geared. My boat is a bit over propped but it seems to do ok.
There are two gearboxes for this engine. One is 2.21:1 and the other is around 2.8:1 if I remember right. Find out which one you have before you make a prop decision.
If you have a feathering prop you can have them repitched for a better bite. If it is a folder then maybe it is a bit small for your application.
I never run my 3gm30f over 3 grand. I don't think I have ever had it at wide open throttle, ever.
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Old 07-01-2012
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Re: Beneteau 36CC does not reach hull speed

Your GPS shows 5.1 Kts. You have a .5 KT wind on the nose. Hull speed is supposed to be 7.48KT.
1st you are mixing apples and oranges. Your GPS will show you speed at a given time over the earth. It is showing you the sum of the efeect of boat performance, wind/wave action and current. It will not show you the isolated hull speed. It is no way a performance instrument. If you want a performance answer you need to measure speed through the water. For this a current on the nose or stern makes no difference. However the wind will. What you do need is something that will measure your speed through the water and have no wind, That will get you very close. If you must rely on GPS then do it without wind and current. The other cheap and accurate way is to throw something that floats in the water with a very low free board at the bow and measure with a stop watch the time it takes to reach the stern. Now you have a simple time/speed/distance formula to work with. Just like they did several hundred years ago. But it works.
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Last edited by jerryrlitton; 07-01-2012 at 02:08 AM.
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Old 07-02-2012
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Re: Beneteau 36CC does not reach hull speed

If you've had the boat 12 years now, what speed was normal until now? Did you just change to the Maxprop? Is that why you are asking?
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