Propeller Dilemma - Hillyard Moonfleet 36

[Hank Moody]

Advice please. I own a Perkins Prima M50 engined 1992 Moonfleet, one of the very rare GRP Hillyards . It needs a new prop and the existing one has been measured as 20x10 RH. This is, apparently, an unusually shallow pitch for a 20 inch prop. T Norris are recommending an 18x12 replacement on the basis of engine and the assumption that the Hurth gearbox is 2:1 reduction (no plate to be found in engine bay to confirm).
I only took the boat out once before the securing nut failed so I have very little reference for performance and have been unable to make contact with any other owners (only 6 made I believe) . She also happens to be the first boat I have owned. We therefore have limited information to be able to spec the replacement.
Any advice would be gratefully received as the boat has now been out of the water for 6 weeks and I'm keen to resolve with the limited information we have, and confess I am not fully aware of the implications of getting it wrong - apart from generally poor performance/economy.
Apologies in advance for any lapses of forum etiquette as this is my first post

 

[Tranona]

Welcome to the forum

Norris will have used a propeller calculator to arrive at that size, which is indeed appropriate. I expect the person who fitted the 20" prop believed that larger diameter, shallow pitch would give better performance when heavily laden. In general larger diameter is "better", but the limitation of propeller tip clearance often comes in.

In terms of performance there is a trade off between pitch and diameter of roughly equal amounts, so 1" les diameter requires 1" more pitch. That is exactly what is happening here - you are going down 2" in diameter and 2" up in pitch, so the effective "size" and the load it puts on the engine will be the same. So go with the recommendation - Norris have been in this business a long time and know their onions.

BTW the actual model of boat is not that critical. The key data required is LWL, displacement, engine power at rated maximum and gearbox reduction which gives prop shaft speed.

Enjoy your new boat. Good choice but as you say few built as it could not compete with the Moodys and Westerlys, never mind the Scandinavian competitors.

 

[dankilb]

Our boat had an M50 - since re-engined with a Beta 50. The previous owner kept the Hurth box (I’ve always presumed 2:1). Beta recommend 19 x 12, so not far off what Norris have said: Seagoing Propellers - Marine Propulsion Engines Beta Marine

As T. says above, all the better if the spec of the boat has been taken into account in calculating the new prop.

 

[Norman_E]

A feathering prop is a very nice addition to any yacht, and has the advantage that you can set the pitch, and alter it until you find the best setting. Something like a Darglow Featherstream, probably 19 inch, but take Darglow's advice, and play with the pitch until you are happy that you have the best setting. The bonus is a small increase in sailing speed, particularly in light winds, where you will sometimes be happy to sail where you otherwise might have motored.

 

[Hank Moody]

Thanks all, that's very helpful. I was rather apprehensive about the M50 but I have to say it runs incredibly well. She was delivered from Torquay to Conwy in March, motored most of the way and didn't miss a beat (lucky for then the nut didn't fail mid Bristol Channel).
As for the model, I think she compares rather well to Moodys and Westerlys but I suspect the Volvo 240 styling might have lost them some clients. On the other hand that does create lots of flat deck area which is handy in port - especially with young kids around.
The feathering prop is a lovely idea, Norman, but was discounted due to cost. I have quite a shopping list and some things have to give. Speed being one of them sadly!
Thanks again, I dare say I shall be back here before long.

 

[Tranona]

The point about competitiveness is that it was a very expensive boat priced above the other 2 brands, but not having the qualities that stood comparison with Scandinavians. Very difficult to break into a market that already has well established brands. Remember the Hillyard brand had been dormant for years and meant little to new buyers. Does not mean it was not a good boat - just did not capture the imagination! You are lucky down the line to get a quality boat without necessarily the price premium.

 

[dankilb]

Thanks all, that's very helpful. I was rather apprehensive about the M50 but I have to say it runs incredibly well. She was delivered from Torquay to Conwy in March, motored most of the way and didn't miss a beat (lucky for then the nut didn't fail mid Bristol Channel).
As for the model, I think she compares rather well to Moodys and Westerlys but I suspect the Volvo 240 styling might have lost them some clients. On the other hand that does create lots of flat deck area which is handy in port - especially with young kids around.
The feathering prop is a lovely idea, Norman, but was discounted due to cost. I have quite a shopping list and some things have to give. Speed being one of them sadly!
Thanks again, I dare say I shall be back here before long.

I notice the vast majority of our model (Jeanneau Voyage) still have their original M50s. Judging by the state of the foam insulation we removed (soaked in litres of oil), and the PO’s log entry(!), they literally blew the old engine up. Otherwise I’m sure it’d still be going strong.

We’d love a folding/feathering prop too but the cost is off-putting for now. If/when we come to replace the prop though, I’d think hard whether it’s worth it longer term.

 

[penfold]

If you can get to the front of the engine with a long ratchet or breaker bar turn the engine over slowly one turn with the throttle lever in forward gear while an assistant watches how far round the prop goes; if it's half a turn you have a 2:1 gearbox. Are you sure about the prop measurement if it seems unusual, measuring props is a quite approximate game? Most props have their spec stamped somewhere on the hub.

 

[Tranona]

no reason to suppose the measurement is incorrect. As in post#2 there is a reasonable explanation for why a 20~ would have been fitted, but the 18" suggested would be more normal. In a sense it does not matter what the original is - it is getting the right size now and Norris will have done the sums.

 

[penfold]

It would be a good idea to confirm the ratio before splurging the cash though.

 

[Tranona]

It would be a good idea to confirm the ratio before splurging the cash though.

Indeed - but as I pointed out the suggested prop will put essentially the same load on the engine - and I would guess that Norris have done their homework before making a recommendation. After all if they get it wrong it is their responsibility.

 

[Slowboat35]

Norris' are unlikely to 'get the calculation wrong', but they may not be too sympathetic if you've agreed with them the 'box is 2:1 and it turns out to be somethng else...

I would suggest though that assuming a gearbox ratio is rather risky, shaft revs are an important factor in propellor's spec.

Fortunately that's easy enough to determine. Fix a mark (paint/tape etc) on the shaft and accurately count how many turns of the engine (using a socket and extension bar w. decompressors open if you have them) it takes for the shaft to achieve exactly one revolution. Find out what other ratios are to be found in that gearbox for self-assurance that you've counted right!

 

[Laminar Flow]

There are other reasons to having a larger dia prop, such as one running in the disturbed flow behind a wide deadwood to get some clearance. In terms of performance, a larger diametre is always preferable to increasing pitch as it keeps the blade loading down. In the OP's case, however, the gearbox has a pretty standard 2:1 ratio and the tip velocity on a 20" diametre starts to become marginal.

On the upside, the smaller prop will benefit sailing performance. A number of Hurth boxes allow for the prop to spin while sailing and this will reduce resistance about by half, i.e 0.4 kts loss of speed versus 0.8 kts (check with the manufacturer or the manual of your gear box).

 

[penfold]

Indeed - but as I pointed out the suggested prop will put essentially the same load on the engine - and I would guess that Norris have done their homework before making a recommendation. After all if they get it wrong it is their responsibility.

I would be very surprised if their advice is not couched explicitly as a conditional recommendation.

 

[Laminar Flow]

I would be very surprised if their advice is not couched explicitly as a conditional recommendation.

Recommendation is easily verified if the OP were to enter his boat data into an online calculator such as VicProp, and even from his couch.