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GreggL · 27-08-2012, 05:39

So i got a 33 footer with a 29hp auxiliary; gets me going at roughly 6 knots without putting the hammer down on the throttle. Supposedly my hull speed it about 7 knots. Would a larger engine speed things up or is it still determined by the theoretical hull speed? Not like I can change anything but just curious.. Thanks!

**skipmac** · 27-08-2012, 05:47

Quote:

Originally Posted by GreggL

So i got a 33 footer with a 29hp auxiliary; gets me going at roughly 6 knots without putting the hammer down on the throttle. Supposedly my hull speed it about 7 knots. Would a larger engine speed things up or is it still determined by the theoretical hull speed? Not like I can change anything but just curious.. Thanks!

In a word, yes. Once you start approaching the boat's hull speed, trying to get another knot or half knot requires more and more horsepower and you will reach a point where you can achieve no more speed unless you have enough power to lift the hull out of the water to plane.

I think 29 HP is a reasonable power for your boat. The only potential benefits that you might see are the ability to maintain cruising speed against a strong head wind or waves and possibly faster acceleration or braking in reverse when docking.

casual · 27-08-2012, 05:54

<LI class=mitem><LI style="DISPLAY: none" class=jsb><LI style="DISPLAY: none" class=jsb>Search Results

Hull speed - Wikipedia, the free encyclopedia

jeremiason · 27-08-2012, 06:22

Before spending $12k or more on a new engine you might want to play around with yours a little. You should eb able to achieve hull speed with your engine.

I assume you have the Yanmar 29 hp which should have max rpms of 3600. Does you current engine reach 3600 rpms or exceed it? It could be that the propeller is pitched wrong or that you need a three blade instead of a two.

When you are measuring your speed, are you doing it as through the water speed or GPS? As you probably know, but many forget, your GPS speed is relative to you movement over ground, which is effeced by current and wind.

Was your bottom clean when you measured the 6 knots? A dirty bottom and/or prop will slow you down drastically (A full knot or better).

Sailmonkey · 27-08-2012, 06:29

Quote:

Originally Posted by jeremiason

Before spending $12k or more on a new engine you might want to play around with yours a little. You should eb able to achieve hull speed with your engine.

He's not even trying to go faster. The OP stated that the 6 knots is easily achieved.

**Dockhead** · 27-08-2012, 06:47

I understood that you're not planning to actually change your engine -- you're just curious.

As others have said, hull speed is hull speed -- laws of physics. Somewhere around hull speed, you boat will squat and starting climbing its own bow wave. Power required goes up steeply. Naturally, fuel consumed also. It's not worth it.

6 knots cruising speed is good for a 33 footer. In my experience, you need to be a knot or so below your hull speed to have good efficiency. In the three years I've had my boat, I have gradually slowed down my cruising speed as I got to know the efficiency characteristics. My hull speed is about 9.3 knots; I now cruise at 7.5 to 8, having reduced my cruising speed RPM from 2800 to 2500 finally to 2250, every time getting big gains in economy and noise for small reductions in speed.

jeremiason · 27-08-2012, 07:31

Quote:

Originally Posted by Sailmonkey

He's not even trying to go faster. The OP stated that the 6 knots is easily achieved.

Opps silly me... your right!

Many people mistake theorectical hull speed as a brickwall, which cannot physically be exceeded, which of course is not true.

There is a great description of Therorectical Hull Speed on Wikipedia...
Hull speed - Wikipedia, the free encyclopedia

Bash · 27-08-2012, 07:37

For the most part, all a larger engine is going to do is make it more expensive to cruise at six knots.

**skipmac** · 27-08-2012, 07:57

Quote:

Originally Posted by jeremiason

Opps silly me... your right!

Many people mistake theorectical hull speed as a brickwall, which cannot physically be exceeded, which of course is not true.

There is a great description of Therorectical Hull Speed on Wikipedia...
Hull speed - Wikipedia, the free encyclopedia

Certainly not a brick wall. I would say more like a soft, foam wall. Initial contact is soft and yielding but the harder you push, the more resistance you will get from the wall.

Excellent article in wikipedia by the way. One very interesting comment in the article, it said that the hull speed concept is not used in modern naval architecture.

Dennis.G · 27-08-2012, 08:16

6 knots under power in a 33' sailboat is good performance.
Be happy.

GreggL · 27-08-2012, 08:58

Quote:

Originally Posted by jeremiason

Before spending $12k or more on a new engine you might want to play around with yours a little. You should eb able to achieve hull speed with your engine.

I assume you have the Yanmar 29 hp which should have max rpms of 3600. Does you current engine reach 3600 rpms or exceed it? It could be that the propeller is pitched wrong or that you need a three blade instead of a two.

When you are measuring your speed, are you doing it as through the water speed or GPS? As you probably know, but many forget, your GPS speed is relative to you movement over ground, which is effeced by current and wind.

Was your bottom clean when you measured the 6 knots? A dirty bottom and/or prop will slow you down drastically (A full knot or better).

Yes I have the 29hp yanmar. I usually run it between 2000 and 2500. This will produce speeds of 5.5 to 6.6 knots respectively. although my throttle will only allow up to 2600 rpm regardless. Is this not normal?

When I calculate speed I favor the gps usually. That is unless I'm going nowhere specific and just like to see my sailing speed over the water.

**skipmac** · 27-08-2012, 09:09

Quote:

Originally Posted by GreggL

Yes I have the 29hp yanmar. I usually run it between 2000 and 2500. This will produce speeds of 5.5 to 6.6 knots respectively. although my throttle will only allow up to 2600 rpm regardless. Is this not normal?

When I calculate speed I favor the gps usually. That is unless I'm going nowhere specific and just like to see my sailing speed over the water.

If you are trying to evaluate engine performance the only thing that matters is speed through the water. The GPS will show you how fast you are getting to your destination but if you have a 2 knot current going with you then you will be going 7.5-8.6 kts. Obviously the engine isn't doing all that.

When you say the throttle only allows up to 2600 rpm can you elaborate? Do you know what the max rated rpm is for the engine? If rated max is 3600 rpm then you may be over-propped.

GreggL · 27-08-2012, 09:34

Quote:

Originally Posted by skipmac

If you are trying to evaluate engine performance the only thing that matters is speed through the water. The GPS will show you how fast you are getting to your destination but if you have a 2 knot current going with you then you will be going 7.5-8.6 kts. Obviously the engine isn't doing all that.

When you say the throttle only allows up to 2600 rpm can you elaborate? Do you know what the max rated rpm is for the engine? If rated max is 3600 rpm then you may be over-propped.

My throttle level fully down equates to a 2600 rpm reading on the gauge.