Might as well re-engine now I guess?

[nimbusgb]

Only scanned some of the thread but here goes.

60 hp in a 37 foot boat? 40 is ample!

£3500 for saloon upholstery is way over the top!

 

[Steve_N]

Only scanned some of the thread but here goes.

60 hp in a 37 foot boat? 40 is ample!

£3500 for saloon upholstery is way over the top!

I think that both of those points have been covered further up, but;
- I've really enjoyed having a big, smooth and lazy (read over-powered) engine and don't want it's replacement to be smaller and working harder at high revs. I prefer soporific to frantic. I know all about being supposed to work a diesel engine hard but I'm prioritising my preferences over those of the new engine and it'll either have to put up or shut up.The last one has put up for 32 years without drama and the new one won't have to last that long so hopefully it'll all be hunky dory.
- I've binned that upholstery quote.

 

[Steve_N]

To update this thread, I've decided to re-engine. On our 'summer' cruise we had a few engine niggles; the circulation pump has developed play in the bearings and a slow drip, there's a small leak from the end of the heat exchanger, the exhaust riser looks like it may be beginning to perforate just underneath the top of the exhaust hose and finally, for the first time ever in all my years of sailing, it failed to start one morning with a fuel system full of air, the lift pump being the prime suspect.

So it's going to be a Beta and although I first thought that it should be the 50 I now thinking that the 43 is a very good match for the outgoing MD21B.
I'd be grateful if those in the know could run an eye over my figures below and see if they agree.
We swing a 17x11 fixed 3 blade prop and the gearbox has a 1.93:1 reduction. We cruise all day at 1800rpm giving 6.1 knots, occasionally at 2000 for 6.4 or in extremis 2800rpm gives hull speed plus a bit and it really starts to dig in. I've never even tried to see what max revs are but in neutral it is governed to 4500.

Reading off the power curve for the MD21B we get:
1800 25bhp
2000 27.5bhp
2800 42bhp

The comparison for the Beta43 is:
1800 30bhp
2000 35bhp
2800 43bhp
so it is producing even a bit more power at the lower revs.

To my novice eyes that looks like a good match and suggests that with the standard 2.00:1 Beta reduction I might expect to use my existing propellor and achieve similar results, or is that too simplistic? Obviously I'll be discussing this with the supplier but I'd welcome opinions in advance. Thanks.

 

[macd]

Steve: 18 moths ago I re-engined with a Beta 50, which I think is a very similar lump to the 43. I'm thoroughly happy with it, but you might care to check with Beta on the one issue I had: the air filter withdraws sideways. This is no problem in itself, but if the engine is a tight fit in its box, as mine was on the starboard side, it's impossible to withdraw the filter (or the entire filter assembly). I overcame this by making a right-angle adaptor. But you may want to check with Beta if space is tight on that side of your engine bay.

 

[Tranona]

To update this thread, I've decided to re-engine. On our 'summer' cruise we had a few engine niggles; the circulation pump has developed play in the bearings and a slow drip, there's a small leak from the end of the heat exchanger, the exhaust riser looks like it may be beginning to perforate just underneath the top of the exhaust hose and finally, for the first time ever in all my years of sailing, it failed to start one morning with a fuel system full of air, the lift pump being the prime suspect.

So it's going to be a Beta and although I first thought that it should be the 50 I now thinking that the 43 is a very good match for the outgoing MD21B.
I'd be grateful if those in the know could run an eye over my figures below and see if they agree.
We swing a 17x11 fixed 3 blade prop and the gearbox has a 1.93:1 reduction. We cruise all day at 1800rpm giving 6.1 knots, occasionally at 2000 for 6.4 or in extremis 2800rpm gives hull speed plus a bit and it really starts to dig in. I've never even tried to see what max revs are but in neutral it is governed to 4500.

Reading off the power curve for the MD21B we get:
1800 25bhp
2000 27.5bhp
2800 42bhp

The comparison for the Beta43 is:
1800 30bhp
2000 35bhp
2800 43bhp
so it is producing even a bit more power at the lower revs.

To my novice eyes that looks like a good match and suggests that with the standard 2.00:1 Beta reduction I might expect to use my existing propellor and achieve similar results, or is that too simplistic? Obviously I'll be discussing this with the supplier but I'd welcome opinions in advance. Thanks.

You should match your propeller to achieve hull speed at close to maximum engine rpm. Not sensible to compare with your existing engine, but should start from scratch with the new engine/box combination. Pretty sure you will have to reverse the rotation of the prop so you need the PRM gearbox that will run either way - unless it is a feathering prop of the type such as a Max Prop that can be assembled to turn either way.

Suggest you put your boat data into the Propcalc programme on www.castlemarine.co.uk to get a recommended size and then see how this compares with your existing prop.

 

[ianj99]

You should match your propeller to achieve hull speed at close to maximum engine rpm. Not sensible to compare with your existing engine, but should start from scratch with the new engine/box combination. Pretty sure you will have to reverse the rotation of the prop so you need the PRM gearbox that will run either way - unless it is a feathering prop of the type such as a Max Prop that can be assembled to turn either way.

Suggest you put your boat data into the Propcalc programme on www.castlemarine.co.uk to get a recommended size and then see how this compares with your existing prop.

You mean maximum horsepower rpm, not the max allowable rpm (red line), which is at 2800rpm according to the OP.
The OP's existing prop is clearly a good match for the Beta 43.
I cannot see how going to the trouble and expense of tweaking the diameter or pitch will be a worthwhile exercise unless sea trials highlight a performance issue.
My MP446 (46bhp @3000rpm, 2:1 PRM) with a 17" Kiwiprop produces comparable results (a bit slower due to the weight and long keel) with the pitch equivalent to 11".

 

[Steve_N]

Thanks for the comments. Am on a 2G mobile signal so apologies for the brevity.

My engine box has opening front and port sides and is tight on the stbd side so, yes, the air filter access may be a problem. Measuring up everything tomorrow. The 43 and 50 are identical externally, just bore/stroke differences I presume.

Regarding LH prop, for the 50 I was recommended the Technodrive hydraulic box but an advantage of going for the 43 instead is that the standard TM60 box can do LH ahead at the 2:1 reduction with the optional oil cooler. Saves $.

 

[Tranona]

You mean maximum horsepower rpm, not the max allowable rpm (red line), which is at 2800rpm according to the OP.

No, I mean the maximum WOT, which is usually the point of maximum output as well, as it is for the engine in question. The engine is governed to 2800 rpm according to the data sheet. So the prop should allow the engine to achieve those revs and achieve hull speed.

Sensible to check in advance whether the existing prop is appropriate rather than just guess.

 

[gandy]

We swing a 17x11 fixed 3 blade prop and the gearbox has a 1.93:1 reduction. We cruise all day at 1800rpm giving 6.1 knots, occasionally at 2000 for 6.4 or in extremis 2800rpm gives hull speed plus a bit and it really starts to dig in. I've never even tried to see what max revs are but in neutral it is governed to 4500.

Reading off the power curve for the MD21B we get:
1800 25bhp
2000 27.5bhp
2800 42bhp

The comparison for the Beta43 is:
1800 30bhp
2000 35bhp
2800 43bhp
so it is producing even a bit more power at the lower revs.

To my novice eyes that looks like a good match and suggests that with the standard 2.00:1 Beta reduction I might expect to use my existing propellor and achieve similar results, or is that too simplistic? Obviously I'll be discussing this with the supplier but I'd welcome opinions in advance. Thanks.

That looks as if the existing prop might be lower pitch than ideal.

Taking the first line, at your previous cruising speed of 1800 rpm that gives a prop speed of 932rpm. To get the same prop speed from the Beta gives an engine speed of 1850rpm.

 

[Steve_N]

That looks as if the existing prop might be lower pitch than ideal.

You may well be right. I tried the Propcalc program (thanks Tranona) and for my current engine it suggests 18x14 rather than the 17x11 that I have and feel is spot on.
For the Beta43 it comes out at 17x14 with a 2:1 ratio, but if I go for the ZF25M (HBW250) gearbox I can get a 1.88:1 ratio for which Propcalc suggests 17x13.

My gut feeling is that the Beta43 with that 1.88:1 ratio will be quite happy with the existing 17x11. Time will tell I guess.

 

[ianj99]

You may well be right. I tried the Propcalc program (thanks Tranona) and for my current engine it suggests 18x14 rather than the 17x11 that I have and feel is spot on.
For the Beta43 it comes out at 17x14 with a 2:1 ratio, but if I go for the ZF25M (HBW250) gearbox I can get a 1.88:1 ratio for which Propcalc suggests 17x13.

My gut feeling is that the Beta43 with that 1.88:1 ratio will be quite happy with the existing 17x11. Time will tell I guess.

As you said you have a 1.93:1 at present, a 1.88:1 is near enough to make no difference (2.6%) so the existing prop will work fine. (you'll do 6.5kts at 2000rpm)

 

[Tranona]

You may well be right. I tried the Propcalc program (thanks Tranona) and for my current engine it suggests 18x14 rather than the 17x11 that I have and feel is spot on.
For the Beta43 it comes out at 17x14 with a 2:1 ratio, but if I go for the ZF25M (HBW250) gearbox I can get a 1.88:1 ratio for which Propcalc suggests 17x13.

My gut feeling is that the Beta43 with that 1.88:1 ratio will be quite happy with the existing 17x11. Time will tell I guess.

If it is a fixed prop you can get it repitched - but probably not to 14". If it is feathering you may be able to adjust the pitch. Better to get the bigger reduction and change the prop if need be. You will be underpropped with an 11" pitch. One inch of pitch is approx 250 rpm, so you will be running at higher rpm for a given boat speed You may well not reach hull speed before you run out of engine revs and your cruising speed will be around 500 rpm higher than necessary. In many ways the prop is more important than the engine/box so better to get that right first.

 

[ianj99]

If it is a fixed prop you can get it repitched - but probably not to 14". If it is feathering you may be able to adjust the pitch. Better to get the bigger reduction and change the prop if need be. You will be underpropped with an 11" pitch. One inch of pitch is approx 250 rpm, so you will be running at higher rpm for a given boat speed You may well not reach hull speed before you run out of engine revs and your cruising speed will be around 500 rpm higher than necessary. In many ways the prop is more important than the engine/box so better to get that right first.

I think you or I have missed something?
His existing prop and 1.93:1 ratio allows full revs and hull speed according to the OP, and also sensible cruising at 1800 or 2000rpm.

"We swing a 17x11 fixed 3 blade prop and the gearbox has a 1.93:1 reduction. We cruise all day at 1800rpm giving 6.1 knots, occasionally at 2000 for 6.4 or in extremis 2800rpm gives hull speed plus a bit and it really starts to dig in. I've never even tried to see what max revs are but in neutral it is governed to 4500".

So surely changing to a 1.88 ratio doesn't justify repitching the prop. Or are you suggesting he's been underpropped all along?

 

[Tranona]

I think you or I have missed something?
His existing prop and 1.93:1 ratio allows full revs and hull speed according to the OP, and also sensible cruising at 1800 or 2000rpm.

"We swing a 17x11 fixed 3 blade prop and the gearbox has a 1.93:1 reduction. We cruise all day at 1800rpm giving 6.1 knots, occasionally at 2000 for 6.4 or in extremis 2800rpm gives hull speed plus a bit and it really starts to dig in. I've never even tried to see what max revs are but in neutral it is governed to 4500".

So surely changing to a 1.88 ratio doesn't justify repitching the prop. Or are you suggesting he's been underpropped all along?

The Propcalc says 17*14. It is irrelevant what his current set up is. Start from a calculated size for the engine/box combination he is buying, not from what he has. If Propcalc is right, even with the lower ratio he will be significantly underpropped. Best to get the size checked by a propeller specialist, but Propcalc is pretty good on a straightforward boat like this and is sufficiently different from the current prop to warrant further investigation. 2" of pitch is 500 rpm at the engine.

 

[ianj99]

The Propcalc says 17*14. It is irrelevant what his current set up is. Start from a calculated size for the engine/box combination he is buying, not from what he has. If Propcalc is right, even with the lower ratio he will be significantly underpropped. Best to get the size checked by a propeller specialist, but Propcalc is pretty good on a straightforward boat like this and is sufficiently different from the current prop to warrant further investigation. 2" of pitch is 500 rpm at the engine.

That's all very well, and as usual, you assume everyone has deep pockets and a money no object approach, but he may be happy with the existing prop, not want to go to the added expense or sea trials / haul out, prop removal, pitch adjustment or new prop, relaunch, more sea trials etc.