New engine or old prop... or something else?

seumask said:
Please note we chose to do this and get to max speed at 2800 revs of the 3500 avaliable.
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This is not a good ideas. The engine is designed to run close to its maximum. Loading it to 2800 may cause serious long term damage to the engine. As said earlier engines should be propped to achieve maximum hull speed at close to rated RPM and cruise at approx 70% of maximum. This requirement is consistent among engine manufacturers so I wonder what you know that leads you to think your strategy is a good idea.

Genuine question as would not want others to do a similar thing without hearing a sound technical explanation.
 
Tranona said:
This is not a good ideas. The engine is designed to run close to its maximum. Loading it to 2800 may cause serious long term damage to the engine. As said earlier engines should be propped to achieve maximum hull speed at close to rated RPM and cruise at approx 70% of maximum. This requirement is consistent among engine manufacturers so I wonder what you know that leads you to think your strategy is a good idea.
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Agreed. It's not uncommon for cruising sailors to overprop a little, partly for fuel economy, partly for a quieter life. But I'd put a limit on that of around 10% (so 3500 down to 3150 on the engine mentioned). This is pretty arbitrary, I know, but realistic. 20% is too much. It varies from engine to engine, but the final 10% of revs doesn't add a great deal of hp, anyway, since that's when the torque's in its steepest decline.

From the OP's point of view there are also warranty issues. The engine supplier is likely to take a very dim view of the degree of overpropping he has. It will probably not meet his specified installation criteria (I assume the OP has an installation manual, even if the work was done by a third party). As an aside, I'm surprised he hasn't mentioned the smoke screen he probably tows at WOT. I imagine the engine would be over-fuelling quite wildly.
 
On my discussions during the install. The installers were aware of the oversized prop and discussed it with me but I was given the impression that it was marginal and would not cause mechanical issues.

As suggested by another user, not being able to achieve hull speed (1 knot less) is probally all the stuff I have stuck to the hull. So it looks like my problem is that I can achieved max speed at around 2400 RPM instead of the max RPM 3200. I also idle along in the marina at 3.5knots!

On the assumption that the current prop was sized correctl for the 3.02:1 gearbox and both new and old engines have a similar HP. Would moving to the 2.63:1 gearbox cause the prop to be marginally oversized or grossly oversized to a point that it may cause possible problems with engine warranty?
 
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seafox67 said:
On my discussions during the install. The installers were aware of the oversized prop and discussed it with me but I was given the impression that it was marginal and would not cause mechanical issues.

On the assumption that the current prop was sized correctl for the 2.63:1 gearbox and both new and old engines have a similar HP. Would moving to the 3.02:1 gearbox cause the prop to be marginally oversized or grossly oversized to a point that it may cause possible problems with engine warranty?
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Is it not the other way round ?

What model was the original engine could be either Md7b or md11
 
Those are reduction ratios.
I believe your engine is rated to 3200rpm.
So:
with the 3.02: box, that equates to 1060rpm at the prop.
With a 2.63:1 box, it equates to 1217rpm at the prop.

Obviously if the prop is spinning that much faster, it will require more hp to drive it. Trouble is, the engine only has so much power, which is why it won't get anywhere near the rated engine rpm. I suspect it may even have been a little overpropped with the old engine/gearbox.

It might have been wise when specifying the engine to go for a ratio more like your old box. But perhaps that wasn't even available? Changing it now might be expensive and troublesome. Probably much cheaper and simpler to change the prop (assuming it's not a featherer/folder).

The prop speed with either ratio, by the way, is well within general norms for a yacht auxilliary.
 
Your problem is similar to trying to pedal a bike up hill in too high a gear you just don’t have enough grunt
You either change the prop or gear box props are cheaper .
 
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macd said:
Those are reduction ratios.
It might have been wise when specifying the engine to go for a ratio more like your old box. But perhaps that wasn't even available? Changing it now might be expensive and troublesome. Probably much cheaper and simpler to change the prop (assuming it's not a featherer/folder).

The prop speed with either ratio, by the way, is well within general norms for a yacht auxilliary.
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Looking back through this thread... I can only blame myself :(
I had a small time slot to get the engine replaced, I got a good quote based on them keeping bits they could use on the new engine. They did suggest (but not insisted) on replacing the prop. I chose to keep the prop to keep the cost down. However, I would have replaced the prop if I had felt at the time it would cause long term damage and warranty issues.

There were issues getting the same gearbox ratio. I went with what was available and recommended.

The prop is just a 2 blade bronze thing but with still paying for the engine I will need to look for the cheapest option.
 
I'm still puzzled as to why, or how, a new engine and gear box can have been installed and signed off without the installer (and customer - the OP) taking any notice of the data currently being quoted. The diagnosis being offered, that the prop needs changed, sounds sound - but why was this not apparent when the engine was run and tested - after installation.

My assumption, is to give the installer the benefit of the doubt - but if the data is correct then this issue should have been raised before the contract to install the engine was even signed and/or should have been raised when the engine was originally commissioned - not over 6 months later.

Jonathan
 
Neeves said:
I'm still puzzled as to why, or how, a new engine and gear box can have been installed and signed off without the installer (and customer - the OP) taking any notice of the data currently being quoted. The diagnosis being offered, that the prop needs changed, sounds sound - but why was this not apparent when the engine was run and tested - after installation.

My assumption, is to give the installer the benefit of the doubt - but if the data is correct then this issue should have been raised before the contract to install the engine was even signed and/or should have been raised when the engine was originally commissioned - not over 6 months later.

Jonathan
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Engine was replaced towards the end of the sailing season.

Probably a combination of roughish seas and I'm a little new to sailing, the sea test appeared to me ok and the warranty was registered.

I did a little sailing/motoring during the winter months and with Scotland getting a little rough weather any performance issues during this time was put down to the weather.

So, It's just been this last month that I have been able to get out in calm waters.
 
I thought I had posted this earlier
You can simply have the prop machined to trim the blades to the correct diam
If you are in Scotland FAL Buckie offer this service
 
As you suggest you are new to all this - Before you rush off and buy a new prop, I note the rushing off part might not quite happen :( ask, again, on here. There are lots of different designs of folding, fixed (number of blades) feathering - you will receive plenty of advice from people who have used their specific design - make the most of the resource.

Props are easy to replace - as long as you can dry out - and I am sure you will receive advise on how to dry out.

Jonathan
 
Thanks to everyone that offered advice to my problem.

After some thinking of what to do next, I've had a proper calculation done for a fixed 3 blade prop. (With quotes over £1000 for a folding prop, they are beyond my budget).
This calculation is Diameter 15.6" with a 11.5" pitch.

I've sourced a new stock (off the shelf) 3 blade prop for just under £300 but it's 16" diameter with a 11" pitch.
This has a very slight measurement difference to the recommended one. Would this margin make much difference to the performance?

I've tried to find out if there is a plus and minus tolerance for props in general. But, I've had not luck.

Cheers
Paul
 
seafox67 said:
Thanks to everyone that offered advice to my problem.

After some thinking of what to do next, I've had a proper calculation done for a fixed 3 blade prop. (With quotes over £1000 for a folding prop, they are beyond my budget).
This calculation is Diameter 15.6" with a 11.5" pitch.

I've sourced a new stock (off the shelf) 3 blade prop for just under £300 but it's 16" diameter with a 11" pitch.
This has a very slight measurement difference to the recommended one. Would this margin make much difference to the performance?

I've tried to find out if there is a plus and minus tolerance for props in general. But, I've had not luck.

Cheers
Paul
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If it’s the correct shaft cone then it’s probably near enough
If you need to alter the taper what would the extra cost be?
How much and how long to wait for made to measure ?
 
scottie said:
If it’s the correct shaft cone then it’s probably near enough
If you need to alter the taper what would the extra cost be?
How much and how long to wait for made to measure ?
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Thanks for your feedback...

The shaft is 25mm which I think is standard.

I've got three possible sources all around £300. The one I am thinking of going for is local and they offered me what sounds like a off shelf stock 16 x 11. My backup source is Fal in Buckie. It sounds like he can make adjustments but there is around a 3-4 week delay for the machine shop. (Machine shop only needed if shaft needs adjusting). I then have a 'last chance' source with someone in southampton.

I've got the boat being lifted out on Monday. So, i'm thinking my best option is to get the old prop off and take it down to Largs to my first choice source and leave the old prop with them. That way they can check the shaft/taper and key stuff before they hand over the new prop to me.

I'm really, really hoping she will be back in the water 1-2 weeks.

Paul
 
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