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post #11 of 22 Old 03-21-2018
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Re: How Fast Should My Boat Go Under Diesel

You should be able to run out to redline in calm seas and wind. That may or may not be close to hull speed. If you can only get 3000 rpm your prop is over pitched. Dialing in less pitch will get you more rpm but may not increae the speed all that much. It should allow you to maintain a higher speed in less than ideal conditions as the engine will be able to rev higher against wind and water. Personally, in a boat like yours, if I could maintain 6 k would be happy and would proably run at 5k for best economy and speed.
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post #12 of 22 Old 03-22-2018
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Re: How Fast Should My Boat Go Under Diesel

Six kts is pretty darned good for a design with a LOT of wetted surface and significant displacement.

As others have noted, keep that large area of bottom clean, very very clean. It is surprising how much a thin layer of slime will slow your boat.
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post #13 of 22 Old 03-22-2018
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Re: How Fast Should My Boat Go Under Diesel

Originally Posted by Baysailor34 View Post
Thank you for the quick responses.

In flat water at slack tide with calm winds I can go 6.0 knots SOG at 3000 RPM. .....
How fast your boat is moving over the ground is meaningless.

Start with the RPMs. You said the motor is rated to max at 3600 and it should get there at wide open throttle, when underway. No matter how fast you're going. How fast you're going can be affected by many other things, but getting to max RPM is a sign of properly drive train configuration and engine health. Improper prop pitch can do harm to a motor over the long term.

While low RPM at WOT is a sign of an over pitched prop (ie there is too much drag on the blades which bog the motor down), the tachnometers on boats can be wildly wrong too. I would get a laser tachometer and confirm those settings on the engine itself. They cost under $20 on Amazon, you just put some reflective tape on a pulley and point the laser at it. Once you know the RPM is correct, then mess with the prop. +/- 100 RPM isn't too bad.

I would also highly recommend calling the manufucturer of MaxProp, PYI. They are extremely helpful and I would want to confirm you have the proper size prop in the first place. If the blade diameter is wrong, fixing it with pitch isn't the right solution, even if it's better for the engine. Who knows if the prior owner fitted it properly, or just bought a used one.

SOG may be a mistatement on your part, but it refers to speed over ground, which is measured by a GPS as your speed over the earth's surface. It's affected by current. Hull speed is measured by speed through the water. You may be making hull speed through the water, even though facing an oncoming current and seeing slower SOG. Speed isn't the issue here. Good luck troubleshooting the RPM.

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post #14 of 22 Old 03-25-2018
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Re: How Fast Should My Boat Go Under Diesel

I had been fighting RPM issues with my own boat and a yanmar 1gm10c saildrive unit. One of the things you can do with the yanmars is breifly run them up to max rpm in neutral with no harm. In fact it is one of the diagnostic tests in their service manuals. So for myself, after replacing the fuel feeder pump and rebuilding the cylinder head, I can now hit 3780 RPM in neutral and 2800 rpm in forward while tied to the dock. I was previously only getting about 2800 rpm in neutral and 2250 in gear.
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post #15 of 22 Old 03-26-2018
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Re: How Fast Should My Boat Go Under Diesel

IIRC the rule of thumb (1.3 or 1.4 times square root of waterline length) is based partly on the fact that a displacement hull will create a trough in the water as it travels. So, there's a bow wave and a stern wave and once you approach 1.4 times etc., the displacement hull has dropped into the trough it has created, so such an extent that it can never get past that, unless it has a planing hull.

Of course if you're underpowered for any reason, you won't get that far even with a tailwind.
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post #16 of 22 Old 07-08-2018
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Re: How Fast Should My Boat Go Under Diesel

As for engine HP to boat, yeah that's alot of variables (even when you take windage and sea conditions out of the equation).

My old Tanzer 7.5m (25ft~), 4000lbs~, shoal keel with a beat up old Honda 10hp outboard would do hullspeed at WOT.

My new Mirage 33 has a 2gmf (13hp) and at 2200rpm it'll hold about 5kts. I didn't want to push it more because as far as I'm concerned I'm happy anytime I can motor 5kts of speed on a boat without flogging the engine.

As far as the RPMs go, that's a great tip on the laser tach, I didn't even know those exist and I will be buying one shortly. On the 2gm engines I have heard the exhaust elbow slowly clogs with soot over the years. Some signs are black on the transom or puffs of soot when throttling up, but there may be no real sign other than loss of RPM. As the engine loses it's ability to exhale via the exhaust it also loses the ability to rev to proper RPM. It may be something to look into.

Some youtube videos, nothing fancy

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post #17 of 22 Old 07-08-2018
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Re: How Fast Should My Boat Go Under Diesel

Iíve got 24hp with the same displacement and waterline. 6.7 knots is as fast as it goes. Cruising at 2/3 throttle gives me 6 kts.

Personally, I think yours is performing well. Finer pitch might get you more rpm, but not more speed, and will likely affect reverse adversely. So I would leave it as it is.

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post #18 of 22 Old 07-08-2018
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Re: How Fast Should My Boat Go Under Diesel

Most sailboats swat at bit more under power than under sail. Some swat a lot. Depends on shaft angle geometry of the stern( rocker, increase in submerged volume etc.). Displacement hulls really feel these effects so speed v hp is very boat specific. Swatting really slows a boat down.
Would suggest yo read some David Gerr before messing with your prop pitch. Thereís a science to this and he has it pegged.
Empirically I know my boat swats under power. I know once I reach a point I get little more in the way of boat speed by increasing rpms. Just more swat. Itís real variable where this point is. Impacting variables seem to be full/near empty fuel/water tanks, sea state, direction relative to swell etc.
Sailing we look to be going high 7s/low 8s. Powering 6.5 to 7.

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post #19 of 22 Old 07-08-2018
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Re: How Fast Should My Boat Go Under Diesel

do you mean squat? If so, as the hull gets to moving hull speed it creates a wave with the length of the LWL. If you push it faster the wave length increases and the boat is then trying to power up over the bow wave with the stern dropping into the trough of the created wave. Moving faster than hull speed the boat feels like its stern is squatting.

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post #20 of 22 Old 07-08-2018
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Re: How Fast Should My Boat Go Under Diesel

A good diesel should run efficiently at 1800 to 2000 rpm regardless of speed. If your boat is set up properly it should do 5 to 6 knots in calm water at that RPM. I never run my engine higher than 2000 rpm. But then again I never have engine problems.

Diesel Mechanic

Last edited by [email protected]; 07-08-2018 at 10:39 PM.

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