Big engine in small sailboat but not much forward motion

[capt jgw]

The main test for proper prop pitch is what RPM's can you reach in forward gear. If you can run right up to or beyond redline, you're under propped. If you can't reach 80% of redline, you're over propped. Either one will cause poor speed under power. As mentioned earlier, a prop shop can usually increase or decrease pitch a couple inches. I had a similar problem with my 28 footer and and 2GM. I was getting about 5 knots at redline, I had the pitch increased by 1 inch and got 6 knots at 90%.
Also the shape of the aperature and rudder can screw up the water flow if they're not done correctly. Not much you can do about that.

[rslifkin]

Originally Posted by capt jgw

Also the shape of the aperature and rudder can screw up the water flow if they're not done correctly. Not much you can do about that.

If that turns out to be an issue, a different prop may help (and it would be time to talk to a good prop guy about what to try). Props with different blade shape and/or a different blade count and different blade area will be affected differently by stuff in front of it causing weird water flow to the prop.

[Lee Jerry]

Originally Posted by capt jgw

The main test for proper prop pitch is what RPM's can you reach in forward gear. If you can run right up to or beyond redline, you're under propped. If you can't reach 80% of redline, you're over propped.

This is wrong. Rated RPM at WOT is the MINIMUM you want to achieve, not the max. No idea where 80% RPM came from. In flat water, with clean hull and prop, you need to be able to reach rated engine speed at wide open throttle.

Also the shape of the aperature and rudder can screw up the water flow if they're not done correctly. Not much you can do about that.

You don't know this. There could be some fairing or something else that could be done to help if this were in fact the issue (or part of it).




To the OP, I think it is unlikely (but not impossible) that the transmission or prop are that far off in match; you should be hitting ~6 kt. So you may want to look elsewhere. For example, is the engine functioning properly? Did you do a compression test? Is the fuel flow good? Maybe it isn't producing the power it is supposed to. Another possibility is that you have a bearing issue(s), either in the engine or along the shaft, that is sucking up the engine power. Do you have any other symptoms, such as excessive noise or vibration or heat? It could be the prop, but IMHO you need to rule out some other possibilities first.

[TrentePieds]

Hugo:

Let's credit the two extra feet the 27 has over the 25 LOA to the LWL. Then your LWL would be 23 feet and your "hull speed" 1.35 x 23^½ = 6½ knots. In reality expect no more than 5½ knots.

Yer engine delivers about 16HP at 3,400 RPM ("Cruising Revs") and about 18 HP "flat out" at 3,600 RPM. Your cruising RPM is very close to your max RPM, so you'd better get used to the racket :-)!

Yer common propeller calculation sez you have to use a 14D x 8P if a 2-blader, a 13D x 8P if a 3-blader and a 12D x 8P if a four-blader. A 4-blader will detract considerably from your sailing performance, the 3-blader less so, and the 2-blader least, most particularly if you can stop it so the blades are up'n'down in the deadwood.

The fact that your gear box is a "V" drive box does not affect any of that.

These sorts of calculations are always just a first step, never quite accurate. So you cannot really know what is going on until you haul the boat and actually check the dimensions of the existing prop. The diameter and pitch should be stamped on the hub.

To re-prop, if required, is relatively cheap.

All the best

TrentePieds

[Kelkara]

Has the OP confirmed that the bottom is clean? Over Christmas I was getting a naximum of 3.0-3.5 knots, but I wasn't going far then, so just lived with it ... This Spring I got a diver to clean my hull, and he found a colony of mussels living on the bottom of my keel (not visible from the surface) ... Made all the difference back to my normal 5kts at 3000rpm, but I prefer to cruise without the engine screaming so do 4kts at 2000rpm. My boat will sail at 6 kts, but never had her motoring that fast.

[jamhass]

How is the engine running? Does it hit full rpm in neutral? Any smoke under load? might not a prop issue at all. Many issues can contribute such as blocked exhaust elbow, dirty injector(s), low compression, etc etc.

[Compass790]

Originally Posted by Ballsnall

If you have a clean bottom and the props not fouled then I would suggest you go back to basics before making any drastic changes.

What is the WOT revs?
What is the gearbox reduction gearing?
What is the prop diameter & pitch?

Without this basic information your just guessing.

What he said in both posts.


Having said this I would be very surprised if there is not something badly wrong if you can only get 3.5knts with that engine.
Our 26ft sailboat does 5knts at 2400 rpm ( its overpropped) with an 8hp yanmar single. It would be lower drag than yrs but you have double the hp.
A few weeks ago we took it out to dive on it & clean the bottom & it would only do about 2knts. The prop had a big oyster on one of the blades & there was barnacles, oysters & small weed on the bottom. After the clean the transformation was instantly noticeable.
My guess is you should be able to get at least 5.5 knts if not 6 knts with clean everything. The pitch & dia is often stamped on prop hub. You can check online on the Vicprop calculator what you should have.

[Searles]

To start with get a hand held taco and measure the shaft speed as it comes out the drive ,but if as expected plus there is a history of this problem ,I would look for a BIG fat apature with verry little room for the prop to work in clear water ,p s not an unusual situation in apature props .⚓️⛵️

[Hugo B]

Here is an update: I dived under the boat this week (freezing cold) and tried to clean the prop, with not that much success. It hard to hold yourself down, hold your breath and then start scrubbing as well! But what I saw is the the prop is not really fouled, so I do not think that is the problem.

I think my next step is get a Yanmar expert in and let him have a look. There seems to be some evidence that a V drive loses more power in transmission than a direct drive, so this might be it. But this weekend I will take the boat out and run the engine at WOT and see what that does.

If there is no solution, I am considering mounting secondary propulsion to the boat, in the form of a 6 or so HP outboard, like the Tohatsu Pro Sail with a high thrust propeller. I know that 8 HP would be better, but then the outboard gets quite heavy to mount.

By itself it will not make the boat go faster of course, but together with the Yanmar, it might make the boat fast enough against a heavy current for example. It also is a good backup of course. The question is whether an inboard and outboard prop will work together well or whether the wash from the Yanmar prop will make the outboard prop useless. Any thoughts on that? Anyone ever done this?

And sorry, another queston: the transmission unit is a Kanzaki and I think it has a gear ratio of 2.62 to 1. Can this be changed? If so, what needs to be done and what is a better ratio to make the boat go faster?

Thanks

[Don C L]

Just as a point of reference, my boat is similar to yours in dimensions and with a Tohatsu 9.8hp outboard with a 4 blade power prop I can go between 5.5 and 6 kts at something between 1/2 to 3/4 throttle. I am not advocating you ditch the inboard though!

[Wotname]

Several posts have asked - what is your current WOT rpm?

This is a critical question and it really needs answering before we can speculate further!

[Ballsnall]

Originally Posted by Hugo B

It hard to hold yourself down, hold your breath and then start scrubbing as well!

Use a weight belt.

I think my next step is get a Yanmar expert in and let him have a look.

Waste of money, you have a prop issue not an engine issue.

There seems to be some evidence that a V drive loses more power in transmission than a direct drive, so this might be it.

A V drive does lose more power in transmission, 0.025% additional power loss. You have a prop or transmission ratio problem not a drive problem.

If there is no solution, I am considering mounting secondary propulsion to the boat, in the form of a 6 or so HP outboard, like the Tohatsu Pro Sail with a high thrust propeller. I know that 8 HP would be better, but then the outboard gets quite heavy to mount.

There is a solution, remove your prop and identify the pitch then run the numbers or get a prop manufacturer to run the numbers for you. You have a fundamental prop issue as confirmed by previous owners.

You need to address the root cause.

And sorry, another queston: the transmission unit is a Kanzaki and I think it has a gear ratio of 2.62 to 1. Can this be changed? If so, what needs to be done and what is a better ratio to make the boat go faster?

You NEED to find out the actual prop pitch and diameter, gear ratio and hp to get the correct answer.

[Wotname]

One data point - I had a 2GM20 in a 30' heavy steel boat (Bollard Starfire) with a V drive. It had no trouble achieving 6+ kts and at WOT of 3,600 rpm when bottom and prop were clean.

You only need a couple barnacles on the prop to destroy its performance.

[gonesail]

did you take the boat out for a test sail before you bought it?

[Searles]

If you were to fit an outboard on the stern ,with the prop in clear water you may understand how poorly you inboard prop works in a stern apature with little access to clear water,but running both together should work ,even just using your inboard rudder and not the outboard to steer .⚓️⛵️