Correct prop size/pitch

[Trintella]

I taking my Trintella 29 back to the yard who supplied her new prop because it seems to be far to highly pitched - e.g. at tick over the boat travels at about 4 knots. at full throttle the boat travels at over 6 knots but the engine is still only firing at about 1000 rpm - perhaps less - if I knock it out of gear the revs sound like they double. She has a newly rebuilt Albin 10hp petrol with a 1:1 ratio gearbox. Max rpm 1600, max torque 1400.

I believe the new prop is 13" diameter with a pitch of 8. The old prop looked smaller, not sure about the pitch. Both twin bladed. Never had the engine running with the old prop as I bought the boat as a 'non-starter'.

As I have never owned a boat with an inboard before, and its so long since I went on any other boats with in boards, what sort of speed should I be aiming for at tick over when in gear & any ideas if 13" 8 pitch sounds anywhere near right for a Trintella 29?

 

[bob_tyler]

If the Albin rebuild came from the boat originally, why was the prop changed?

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[Trintella]

Sorry - shoud have said, old prop de-zinc'd


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[Bejasus]

On your specs, the program I just ran, came up with 12x10 which is fairly equivalent to your 13x8.

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[bob_tyler]

But why change the spec? If the original had been OK, and there was no suspicion about that, a new duplicate should have been fitted.

Make the yard change it or perhaps refit the de-zinced prop. if it wasn't too bad, and try it out before fitting a new one.

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[Trintella]

The yard has kindly agreed to lift her foc in slings next week so I can change to the old prop (and double check the new one is the right one).

I doubt I have any recourse to them if 13" 8 pitch is reasonable given the figures - I should have insisted on an exact duplicate of the old one at the time I suppose (and come on here to ask these questions before rather than after the event!)

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[Paragon]

This program you ran, can it be run for any vessel given type/engine etc? If so any chance you could do one for me?

Many thanks

Regards

John

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[Bejasus]

Sure, I'm no expert on the subject but when I have run it and then spoken to prop makers, their answer has invariably been the same.

I need the following for the most detailed feedback, but can get away with less.
1. No. of engines
2. HP per engine
3. Max RPM
4 Gearbox Ratio
5. No. of Bearings(i.e. stern tube & P bracket
6. Weight
7. Length at waterline
8. Beam
9. Draft
10. Hull type(Disp.,Semi-Disp., Planing
11 Max prop size

Failing this, min info would be
No. of engines
HP
Max Rpm
Drive
Weight
Type of hull
LWL


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[Bejasus]

Actually, if it's for the Fisher 25 from your profile, then

LWL = 21'
Beam = 9'4"
Draft = 3'9"
Weight = 10,080 lbs
Engine = 27 HP Yanmar(if original is fitted)
Gearbox ratio = 3.2:1(again if original)
Max RPM = 3600(I am not sure if this is correct




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[Paragon]

Great things these profiles, details correct other than she's being re-engined in a couple of weeks with a Beta BD1005 (28hp) with max rpm of 3600 and a gearbox ratio of 2.94:1, the current prop is 17x10 but I think she would take something a little large if that improved things at all!

I look forward to the result.

Regards

John

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[Bejasus]

Ok, your theoretical max hull speed is 6.87kts, which you cannot achieve with the engine/gearbox you are using. However, a more realistic speed would be 6.5 kts, which theoretically you can do. To acheive this with the set up you suggest, and with a weight of 11000 lbs which is very conservatively above the manufacturers figure for an empty boat, I get the following results from 2 programs.

1. 17" x 14 " (simple Program)

or

2. 17"x10" (more detailed results)
DAR = 33%
Shaft Dia.= 1"
Spacing between bearings = 4'4.5"
Bollard Pull = 711 lbs
Shaft HP = 26
Prop RPM = 1224
Prop Torque = 113 ft.lbs.

This is with a Tobin Bronze 3 Blade Turbine Prop.
Alternative sizes are ..... if a 2 blade prop - 18"x10"......if a 4 blade prop - 16"x10"


As I said, I am not an expert but these results have usually configured well with Prop suppliers recommendations.

Hope this helps


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[Paragon]

George, your a star, many thanks for doing this.

Regards

John

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[Trintella]

I've found the invoice now, the new prop is 14" diameter, pitch 10 !

Surely a reputable yard should have questioned this for a 29ft boat with a 10hp engine ?

Anyone out there got a prop this size with same sort of boat size/engine hp ?

Many thanks for the replies.

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