Two boats virtually identical - one of them 5 knots faster at WOT

[ari]

I am not making any assumptions yet............Lets try and get a reasonably accurate model of the propeller demand curve for OP's vessel, I hate banging on about this but here we go again 'propellers move boats' the engines merely turn them.

Do you have any issues with the SWAG numbers I have created so far?

Can we not just keep it really really simple and look at the fact that he's running out of revs at 3,900rpm at 26 knots and his mates identical boat with the same (but slightly less powerful) engines is hitting 31 knots at the same revs?

 

[MarieK]

I can see this is going to go round in circles and its my fault for not having all the data to hand yet. Pause this for now until I can "try" and get as accurate data as possible.

 

[ari]

The boats are identical, the engines are identical (except yours are slightly more powerful). With the best will in the world there isn't going to be enough weight difference to lose 5 knots and even if there were you'd be losing revs with it.

Step one - swap the props with your mates boat.

If that doesn't work look at gearbox gearing.

Good luck and let us know how you get on.

 

[Solitaire]

Maybe your mate is just a better helm!

 

[gordmac]

Is it not the case that if the same props are turned at the same rpm then the boats will have the same speed? Everything else is irrelevant. The props are different and/or the rpms are different, there is no other valid explanation.

 

[Deleted User YDKXO]

Any progress on this interesting thread?

 

[MapisM]

Is it not the case that if the same props are turned at the same rpm then the boats will have the same speed? Everything else is irrelevant. The props are different and/or the rpms are different, there is no other valid explanation.

LOL, exactly the point I tried to make with post #3!

+1 to Deleted User curiosity, anyway.

 

[MarieK]

Both boats are due to be lifted mid August for a scrub I have a local prop guy coming down to take a look - will report back then.

 

[A_8]

Can we not just keep it really really simple and look at the fact that he's running out of revs at 3,900rpm at 26 knots and his mates identical boat with the same (but slightly less powerful) engines is hitting 31 knots at the same revs?

Very true and well put but for some reason it escapes contributors on this thread in spite it being stated many times by some of us.

 

[ari]

Both boats are due to be lifted mid August for a scrub I have a local prop guy coming down to take a look - will report back then.

How did you get on?

 

[tom52]

What about the trim of the boats ? How their load is distributed and the set of their trim tabs. Surely these factors can account for a 5 knot variation ?