Boat Speed

P

philfin

Well-Known Member
Joined
2 Mar 2006
Messages
69
Location
Med
Visit site
I have an Oceanis 430 with a perkins 4108 engine. With a clean hull I only make about 5.5kts at 2,400rpm. I can increase revs without any strain whatsoever but it makes little difference to the speed. I have a 3 blade feathering propellor. At no point is the engine ever laboured and will allways rev and spin the prop freely.

She can make 8kts on a reach in a good F4, so its not a question of reaching hull speed.

I suspect my prop is just too small. Would welcome your thoughts and advice as to what size prop I should have.
 
http://www.rbbi.com/folders/prop/propcalc.htm
will give you a link to a page that refers to other links for calculating prop size.
I would say you are right. Assuming your gearbox ratio is OK and not slipping, your existing prop is too small. I would seriously recommend Bruntons Autoprop as a replacement. It is expensive but will adjust to the required pitch for the conditions automatically. Bruntons are also very good at sizing right first time but if not right will change it if you are not satisfied. I have a 36ft cat with a little wing engine in addition to the main. Its only 10HP and could only manage to push me along at 3 knots before with a 2 blade folding prop. Now it does 4-5. Remarkable things!
 
I agree...
I'd say she is under-propped.. If she will happily climb to maximum revs with little 'restriction' then that would sound the likely culprit.
 
If your prop is properly absorbing power, and the gearbox is OK, at full throttle in gear when static you'll not quite reach your max revs (3,600 or 4,000, depending on governor setting) and you'll probably see some black smoke. In this case, you need to check with some external source whether your chosen ratio of pitch to prop diameter is suitable. It'll help if you tell us your gearox ratio. Mine was a Hurth 1:1.86 driving a 17 inch diameter prop (pitch unknown - feathering prop) and a 4,000 governed rpm.

If you're easily reaching governed rpm with no black smoke and part throttle when in gear and static, either your gearbox clutches are slipping (depends on type of gearbox) or you're underpropped in either diameter or pitch or both.

So, think gearbox (if it's a Hurth) as well as prop. It's not often boats are turned out with badly mis-matching props . . .
 
[ QUOTE ]
So, think gearbox (if it's a Hurth) as well as prop. It's not often boats are turned out with badly mis-matching props . . .


[/ QUOTE ]

The fact that it is a feathering prop suggests that it is a later addition. Some feathering props like the Maxprop are adustable for pitch so it might be relatively easy to pitch up, albeit needing a lift out. The advantage of Autoprop feathering props is that they self pitch to suit the occasion so unless the diameter was totally incorrect (not at all likely) they will work very well. We know two Maxprop owners locally that had several lift-outs each to repitch before finally finding the right setting.
 
Thanks for all the advice. I hadn't considered the gearbox, though I'm not sure how to tell if its slipping or not?

I suspect the prop because the fellow who had the boat before me had replaced it. Any ideas about what diameter it should be? I will be lifting the boat shortly for antifouling etc and would like to get it sorted in one hit.
 
change the pitch of your prop before trying anything else. Very unlikely that you have a gearbox problem.
A feathering prop is usually adjustable. If too fine a pitch, the engine revs to max but speed isn't great. If too coarse a pitch, the engine will struggle to reach max revs. check the maxprop website to get an understanding of what is involved in changing the pitch.
Not too hard if you are reasonably mechanically minded, but remember to re-fit the blades in the feathering position when reassembling.
http://www.pyiinc.com/index.php?section=...lation&sn=5
 
I now have ascertained that it is a Maxprop and I have a strong suspicion that it is the pitch that needs adjusting. In any case that will be my first attack. Don't trust myself to do it though so will find a decent prop man to do adjustment
 
As I said, my feathering prop was 17 inch with a 4.108 and a Hurth gearbox at (ratio above. memory gone).

Small pitch changes of about 2 degrees made a big difference to the revs achievable (about 500rpm). I don't know how the Maxrop does its pitch control (I don't think the older ones had a variable stop) but mine was a straight forward lockable set screw. Whatever, the manufacturers will provide a manual showing how to make the adjustments, and should give you a clue as to how much change to make. Unless your yard has experience with this prop, it's quite likely you'll know as much as they do when it comes to making such adjustments.

All the besst with it!
 
[ QUOTE ]
Not enough tide to dry out in Poole?

[/ QUOTE ]

No, 2m range on springs and no drying piles, most people go to the Solent for a bit more range. Club boats though so slightly less expensive boat lift, just irritating not to be able to get it right 1st, 2nd even 3rd try!
 
Perhaps as a better idea of where you should be... and as a target speed...

I have an Oceanis 400, with a Perkins Prima....

At 2,400 with a 3 blade fixed prop, we are doing about 7.4kts..... and at full revs (about 3,600) we can just nudge 9kts, but its really really thrashing her!
 
Hi, I have had a simlar problem. My boat has been re-engined with a Nanni engine and Hurth gearbox ( by the previous owner), the engine will hit maximum revs with no effort and boat speed is less than I expected "Its under propped" I thought so started looking into larger/coarser pitch props. I did all the prop calculations and the original prop was the size specified (I even spoke to the Nanni dealers, Peachments) and everyone said that the prop was the correct size. When I looked at the gearbox data plate and checked it against the Hurth manual, I realised that the prop was the wrong hand! It was right-hand but should have been left. My gearbox (HWB50) has different ratios for forward and reverse, 2.15 and 2.72 respectivley. Luckily, I hadn't bought a new right -hand prop, so I just need to get a left-hand prop of the original size. I have looked at the control lever and the gearshift cable can be moved to the other side of the linkage to give forward gear when the lever is moved forward.

Peter
 
From what you have said I would expect 16 inch minimum dia. I think the 17inch mentioned elsewhere is possibly a good indicator of the optimum. Feathering props tend to be bigger dia to achieve the same blade size as a fixed prop because the hub is bigger. However, pitch is probably more important and I would expect to set that at about 14-15 to start. Thats just a guess though. Those things are usually easy enough to adjust following instructions. I would tend (especially if its a smaller dia) to set the pitch to maximum (17-18??) at first to see if you can get the engine to labour. If at this kind of setting it still won't then you need to bite the bullet and buy a bigger prop. If (hopefully) it will then with a very course pitch labouring and won't reach max revs, wind it back in pitch till it does not labour. That's your optimum setting.
 
You must log in or register to reply here.

Share this page

Top