Prop Size and Pitch?

[ciclon1942]

Hi

with the kind help of a forum member ( r.furborough) we have worked out what the correct prop we should have which is a 18" Dia x 11" pitch, if possible I would like to know how a reduction in Dia would affect the pitch for example what pitch would we need if we could only get a 16 or 17" prop to fit.

[ciclon1942]

I have just spoken to someone in Turkey about our prop, seems that the prop we have got is a Michigan MP 18 LH 10 we also have a two blade 18 LH 11.

I am unsure of how how a reduction in pitch from the recommended 18 11 would effect our performance? we have a very long keel on a heavy displacement hull and whilst we have always struggled to go astern we also struggle to windward in anythigs other than calm sea's/wind.

We have a westerbeke 4-107 coupled up to a HBW10 gearbox, the engine seems fine but does not like to rev much more than 2100RPM and there is no noticable increase in speed past 1600RPM so because of this I never rev her past 1800RPM. When we are going into say 20 Knots our speed will reduce from say 5.5 to about 3. It is the marked reduction in speed that worries me due to having problems escaping a lea shore in difficult conditions.

We have not had the boat that long and so are still comming to terms with how things should work.

Any help will be much appreciated as this is a subject that I know little about.

[svcattales]

Originally Posted by ciclon1942

Hi

with the kind help of a forum member ( r.furborough) we have worked out what the correct prop we should have which is a 18" Dia x 11" pitch, if possible I would like to know how a reduction in Dia would affect the pitch for example what pitch would we need if we could only get a 16 or 17" prop to fit.

Hi Ciclon, As a general rule, larger diameter is more efficient. I had to have a prop trimmed to a smaller diameter because it was too close to the hull and there was interference. When you reduce the diameter, you would increase the pitch. I don't know the formula for how much, but a good prop shop should be able to figure it out for you. Ideally, a correct prop should allow your engine to just reach max RPM without exceeding it. Max RPM would be achieved slowly while underway. Hope this helps.

[svcattales]

I just saw your second post. A two blade is technically more effiecient, but I would stick with a 3-blade if you are concerned about powering into seas. A 2-blade might give you higher speed in calm conditions, but a 3-blade will "bite" better and give more power for punching thru seas and headwinds. You mentioned you can only reach 2100 RPMs. If that is below your rated RPM for your engine, you need to reduce pitch which will allow the engine to develop full horsepower and RPM. Most props can be repitched a couple of degrees by a propeller shop.

[GordMay]

An approximate rule of thumb suggests that 1" of diameter absorbs the torque of about 2-3" of pitch, and vice versa.
Each inch of pitch is worth about 150-200 RPM. Increasing pitch reduces RPM.

Reducing your ideal prop’ diameter by 1" might require an increase of about 2" of pitch.

See also “The Propeller Handbook” ~ by Dave Gerr
The Complete Reference for Choosing, Installing and Understanding Boat Propellers
➥ The Propeller Handbook: The Complete ... - Google Book Search

[amarinesurveyor]

The Westerbeke 4-107 should rev out at 3000 rpm while in gear. If you are only getting 2100 rpm now, you would have to take about 4 or 5 inches of pitch out of the prop; the rule of thumb is 200 rpm per inch of pitch. I'm not sure a prop shop will be able to take that much pitch out of your prop, you may have to get a new one.
I agree with cat tales that you should stick with a 3 bladed prop for punching through head seas, and you should go with the largest diameter that will fit.
You should also check that your throttle cable is pushing the throttle on the engine all the way up and that your fuel filters aren't clogged.
Good luck, Brian

[Wainui]

The 4108/107 Max continous RPM is 3,000. You should be able to reach this in gear. Cruising RPM is around 2,400.
My boat is 25,000lbs with a 4108
3 blade 17'' is pitched at 6.8''
I get 6.5 knts @2,400 and 7.2 @2,800
Good luck with this
Mark

[Boracay]

The reduction ration in your gearbox also plays an important part.

Can you tell us what it is? Should be on the maker's plate.

A quick search suggests that the 4107 has only 36hp(?) and that it is an old engine "easily" upgraded to the 4108.

Your boat "looks" like it could be quite heavy so the larger engine may be better than repitching the prop.

Sure is a pretty boat.

[amarinesurveyor]

Originally Posted by Boracay

A quick search suggests that the 4107 has only 36hp(?) and that it is an old engine "easily" upgraded to the 4108.

The 4-107 and 4-108 are essentially the same engine, 40 hp @ 3000 rpm, no need to suggest repalcing it with the 4-108. I believe the 107 & 108 refer to the total cubic inches, 107 versus 108, not much difference; that's why I say they are essentially the same engine. Correct me if I'm wrong. The perkins was also available in 45 hp @ 3500 rpm and 50 hp @ 4000 rpm, but changing the engine out should not be necessary, as long as this engine is in good health.
Brian

[ciclon1942]

The engine has good oil pressure and burns no oil and although by todays standards she is under powered I am reluctant to replace a good engine with something new because it generats ten more HP.

Not sure if I have it right but by reducing the pitch we will be able to increase the revs? I am not there at the moment but will be gojng in a week or so, I will look at the cables and see if they are allowing the engine to rev out or restricting the engine in some way and then take it from there.

As I said my main concern is that she is very slow to windward and that is reduced further when punching into a sea, I know that at near 16 tonnes she is not going to be a race horse but am hoping to improve things a bit. A good prop shop could change the pitch of the prop a degree or so either way, its just that at the moment I am not sure which way to turn.

Thanks for all of your help

Ross

[r.furborough]

Ross,
I have replied to your PM

The fact you have a LH propeller tells me the transmission is being operated in Pos B (reverse) for forward and not Pos A the correct position for forward. This also means the transmission ratio will also be different. I cannot find a pecification for an HBW10 from this era thet telles me what the ratios are for Pos A and Pos B (Hurth/ZF terminology).

Bob

[ciclon1942]

Thanks for the PM Robert, so let me get this right, we could have a LH throw prop working in position 'B' which is a ratio of 1.864:1 instead of the position 'A' 2.045:1 would that cause us the problems we are encountering poor performance, not being able to rev etc.

I would think that this will affect the life of the gearbox also?

Do Hurth/ZF produce a gearbox which would rotate counter clockwise in position 'A' as I am trying to work out why anyone would put a LH throw prop on and then use the ratio which is not for continuous running, when the natural thing to do would be RH and position'A'

The next question seems obvious to me but I do not know, is it a matter of getting a 18 RH 11 prop and changing the morse linkages around? and would this increase our speed ahead?

Regards

Ross

[Extemporaneous]

Don't know the Trans Model #'s, but it wouldn't be a V-Drive would it?

Extemp.

[r.furborough]

Ross,

The lesser reduction ratio means that the engine should rev a lot easier. From our PM's and what you are reporting I think something strange has happened in the past. The propeller may have been modified, normally they would be remarked, but this may not have happened.

I think you may have to check it out thoroughly when you get to Turkey.

I'll be siling the next couple of days and I'll respond to your PM then.

Bob

[ciclon1942]

Hi,

I have looked at the manual for a Westerbeke 40, looks like the engine runs clockwise at the front pulley. If that is the case the rotation at the gearbox end would be counter clockwise? if the engine is rotating counter clockwise and the gearbox rotates the same way when put in the forward position (position'A') the coupling would spin counter clockwise in the ahead position? If that is right then it would be right to assume that a Left Hand throw prop is the right prop to have on the boat?

I am not sure if a standard HBW100 rotates the same way as the engine if you have selected ahead (position 'A')and I am unsure if the hbw100 coresponds to a ZF12M, if any forum member can answer any of our questions it would be much appreciated.

Regards

Ross