Moody 333: Prop size?

[Goldie]

Posted on behalf of a friend with a recently acquired (and very nice!) Moody 333.

The performance under power is disappointing. With a good (and now totally rebuilt) Thorneycroft engine producing around 35 h.p. the boat only reaches around 4.5 kts (verified by GPS) at 1800 rpm, and that with a clean bottom. 1800 is a comfortable sounding rpm for cruising. The prop fitted (3 blades) apparently looks quite small (I don’t have the size confirmed and haven’t seen it but it’s supposedly 14” x 12”). The owner is considering a prop change and is seeking advice, but if any of you with experience of this engine in a Moody 33/333/33S can advise of a suitable prop size - or indeed, confirm that current performance is the norm - it would be very much appreciated, thank you.

 

[robmcg]

1800rpm is too low for cruising revs, it should be 2300 -2400. You are also limited on the Moody 33 hull with prop clearance. Perhaps some new acoustic engine insulation will help making higher revs more tolerable.

 

[Pasarell]

Agree with Robmcg but I wonder if the prop is too big and he's getting turbulence around the hull which reduces efficiency? IIRC on my old Moody 33 the original prop was 2 blade 12" diameter and I changed to 3 blade 13" which made a huge improvement. I could have, just about, got a 14" diameter on her but with virtually no clearance at the hull. Best to talk to one of the prop makers as they're usually very helpful.
For what it's worth I think you will find the Thornycroft T90 is only rated at about 27HP in these boats. Very confusing but in the literature I had the engine was capable of 39HP but only delivered 27HP in that set up.
Hopefully someone with a M33 now will be along soon.

 

[Tranona]

You can't tell if it is the right size without knowing the details of the engine and gearbox ratio. However the speed of 4.5 knots at 1800 rpm sounds about right.

First thing to do is carry out some test runs at 200 rpm intervals from 1600 up to the maximum you can achieve. I expect the engine is rated to at least 3000 rpm (it varies according to the particular version you have). You can check the maximum by revving it in neutral to maximum. You can't do any damage as the governor will cut it off at the rated maximum. You should be able to achieve within 200 rpm of this in flat water if the prop is correctly sized. I would expect a speed of around 6.5 knots at maximum and 5.5 knots at a cruising speed of 2200 rpm.

You can also check the theoretical ideal prop by putting your boat's data into Propcalc on www.castlemarine.co.uk.

If I remember rightly the 333 does not have the restriction on prop diameter that the earlier 33 has and a 14" sounds plausible, although it might be better to go up to a 15" if you have enough clearance.

 

[rudolph_hart]

If it's any help, I have a Dehler 35 (6 metric tons dry) with a 27HP Yanmar, so similar to yours.

Original prop was Volvo 3-blade folder 17" dia x 12" pitch, which had ample 'grunt', both ahead & astern.

Replaced with Bruntons Varifold 3-blade, 16" x 12", which gave slightly more grunt ahead , but is a bit lacking astern. So Bruntons are changing blades to 17" dia (same pitch) which will should give ample bi-directional grunt.

 

[Goldie]

Thanks for all the suggestions and advice so far. I forgot to mention that the owner is an A1 marine engineer so there are no worries with the engine itself.

Thanks for the propcalc link Tranona, I’ll suggest he looks at that as a starter.

It seems to me that the way ahead is either use more revs or look at the prop.

Any input from anyone with first hand knowledge of the 333/33/33S and suitable propellers and performance expectations would be particularly appreciated.

Thanks all, fair winds.

 

[Tranona]

Using more revs is definitely required. 1800 is too low. The prop size sounds about right and would give 4.5 knots in flat water. Props are sized to achieve hull speed at near maximum rated revs which then gives a comfortable cruising speed at around 60-70%, which on that size boat (similar to mine) is 5.5 knots at about 2200.

 

[PetiteFleur]

I have a Moody 33mk2 and have a 13" x 10 3 blade prop with a Beta 25hp engine and 2:1 reduction g/bpx.(actually slightly over propped but not got round to having it re-pitched...). Not sure if the 333 has the same prop shaft angle but 13" diameter is the maximum on the M33, and this in theory does not give quite the recommended prop/hull clearance but is fine in practice. Check the gearbox ratio - in the M33 is was a 1:1 ratio so no reduction; originally fitted with a 2 blade 11" diameter prop.
Get your friend to join the MOA - Moody Owners Association, only a few £'s per year and well worth it just for the technical info available. And if you post this query you'll get an answer very quickly(and there may even be an existing answer in the technical forum).

 

[Goldie]

Many thanks for the response PetiteFleur, and that suggestion has now been made.

Fair Winds.