which prop....again!

[dento29]

Hi all, been refitting my boat for a year or so now (She was just a beaten up hull,) i'm down to the question of which prop...

The hull is a planing hull, dory style 20ft over all with 17ft in the water..beam is 10ft.
she weighs about 2tonne (not sure what to put for fuel and four people)
and i've got a mercruiser 200MR 5L 89' in her
(rpm 4200-4600..stamped on side of engine) with a alpha leg (1.65:1 )

Been on a couple online calculation sites but getting different results 15" to 16" diam and 17" to 19" pitch with speeds of around 35 knots...does this sound right?

Many thanks Paul

 

[QC]

Re: which prop....again!

Hi all, been refitting my boat for a year or so now (She was just a beaten up hull,) i'm down to the question of which prop.

Well that's good!

Is there any way you can beg, borrow or steal a friend's just to get some base numbers? I don't think the calcs are that far off. Are you sure about the weight? 4400 lbs sounds maybe high for a 20 footer, but not too far off depending on expected load. I don't think an MR will clear a 16" dia. prop I would expect a 14.5 x 19 if the engine is running right. If your weight is right, then maybe the 17. You need two anyway, so why not buy one or the other and if it's right, drop by the casino tonight because your luck is running good. If it's wrong, that one becomes the spare, and you use the next one. With the data from the first one we can get it close.

 

[dento29]

Re: which prop....again!

Thank you so much!
once she's painted i'll post some pics..

she has a very thick lay up and i've added weight with a cabin..had her crained into the water so i now how heavy she is, would say about 2tonne with fuel n 4 people.

don't know anyone in my area to brough one but i'm gonna look out for a second hand 14"/14.5", 17" pitch and go from there!!!

 

[hwsiii]

Re: which prop....again!

Dento, with the numbers you gave here are my recommendations from my program.


Boat and Motor

Prop Picker

H

 

[dento29]

Re: which prop....again!

Thanks for the help!
so pitch of 16" ...that three different pitches 16", 17" and 19" i have to work with.
Does the diameter matter? 14" or 15" sound right and is there minimum distance between the top of the prop and the bottom of the calvitation plate..with 15" theres just under half an inch ..ish gap.

Many thanks Paul

 

[hwsiii]

Re: which prop....again!

I woyld go with a larger diameter beause of the load you are having to push, you mat even fin d that you will have to go to a regressive pith prop. And that is 200 HP you have right.

H

 

[dento29]

Re: which prop....again!

Hi, yes 200hp...well i could probably fit a 15" diameter prop but i'm worried that there is a minimum distance from the top of the prop and the cavitation plate? so 17" pitch would take it more to max 4600 rpm and 19" pitch would take it to more like 4200 rpm's? ..am i on the right lines ish??

Whats a regressive pith prop? ..and would a 4 blade prop help?

 

[junior1113]

Re: which prop....again!

we need to get some true #'s on this setup i would go buy a 4 blade 15" pitch and see, bet it works out perfect. also you can run a 15" diameter on your out drive. four blades carry heavy loads more easily on the engine. my 2 cents

 

[dento29]

Re: which prop....again!

Just an update.... got a prop today, four blade solas 19" pitch but only 12.5" diam.
The boat seemed better but w.o.t was 4200rpm
(don't sure of the speed but didn't fell very fast))
and i'd like 4600rpm as max rpm for wot..
also the boat didn't seem to want to get on the plane, very heavy in the arse and that was with the leg right down with the tilt n trim!.... any advice would be helpful.

Many thanks

Spec:

20ft dory hull with cabin..weight is about 2t
mercruiser 200mr
1.65:1 gear...which has blown, if i change the gearing would this help and can it be done???????

 

[hwsiii]

Re: which prop....again!

Dento, you need to WEIGH the boat so we have a real weight to work with. Why didn't you bother getting a speed when you tested the prop, how do you expect to figure what you need if you don't have something to use as a basis.
I am not trying to be hard, but in my opinion you are approaching how to prop your boat in a manner that is going to be expensive and time consuming when you don't get concrete numbers to work with. Nobody can help much without having real data, and at this point you have none. You tried a 12.5" diameter prop on a veryy heavy boat with an alpha outdrive, that certainly is not close to what you need, but you didn't get even a speed reading.
We want to help you, but data is what gets you to the right prop, otherwise you are just trying props til you get lucky.
I Really hope you take this as I intend it to be, a guide to finding the right prop for your boat with the least amount of effort that has to be done to get there.


H

 

[dento29]

Re: which prop....again!

No thanks, you are right. The reason for not getting a speed reading is no speedo or gps.. but i will get friend to lend me his gps or see if he can take his boat out next to mine in order to get a reading.
As for weighting to boat..not sure, i had it crained a couple of years ago and he said about 2 tonnes, without fuel and people. does seem heavy though!
Once again thank you for help and i will try and get some numbers for you.

Many thanks Paul

 

[hwsiii]

Re: which prop....again!

I will be more than happy to help you as much as I can Paul.


H