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Old 22 February 2010, 03:00   #1
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Which Prop do you think please?

Hi Guys

so, i now have a rather splendid Yamaha 90hp two stroke engine on my dory. (thanks Dan! )
(keep me in mind when you ready to change again LOL!)

anyway, the issue i am trying to tweak is the speed when fully laiden.

the boat is heavy, 17.5ft Dory about 800kgs.

and when its just me and three passengers she slips up onto the plane no problem, and tootles along at 30 mph, great!

but when i swap the 'cargo' for 4 burly divers and all their gear - probably and increase of 200kgs more, i drop to about 12mph and she wont quite get on the plane. bad news, huge fuel difference, and speed of delivery to site etc.

anyway, the prop currently is 13.5 x 15. gear ratio 2.00 : 1, revs 5500 max.


so should i change the prop? if so, what would you recommend?

or would hydrofoils do the trick? and again if so, which ones?


i dont think i am far away from cracking the issue, but would be very grateful for any tips


thanks in advance
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Old 22 February 2010, 07:47   #2
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I think the question that needs to be answered is what revs when going 30mph ( is that the 5500 ?) & what revs at 12 mph ?

From this the prop change can be considered ( I think ) .
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Old 22 February 2010, 07:50   #3
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It sounds like you need a finer pitch prop when carrying the divers.
What rpm are you pulling at WOT (full pelt) in each of these scenarios?


Edit: Pete beat me to it!
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Old 22 February 2010, 07:54   #4
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Quote:
Originally Posted by Downhilldai View Post
It sounds like you need a finer pitch prop when carrying the divers.
What rpm are you pulling at WOT (full pelt) in each of these scenarios?


Edit: Pete beat me to it!
Bored at lunchtime - in fact bored at work full stop !
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Old 22 February 2010, 08:09   #5
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In truth, I am not sure as I don't have a rev counter
but I was wot at 30, so I am presuming 5000revs
the engine sounded comfortable with the speed etc if you get my meaning

when "fully laden" the revs were much lower

hope that helps!
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Old 22 February 2010, 08:40   #6
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A lower pitch needed then. Its like trying to pull away in 5th grar for the motor when you are fully laden - it just cant do it ! However drop the gear/lower the pitch and it will rev easier - get you going , but will not hit as high a top speed .

So if you have 15 inch pitch at the mo see if you can borrow a 14 or 13 and see how you get on ?

Dont think the addition of foils will do much for you as you dont mention any excessive bow lift & you can stick divers where you tell them towards the bow for balance !

Actually thinking about it you may also get away with a higher pitch at 'empty' as 30mph seems a little slow for a 90 on flat'ish' dori hull( even a heavy one ). You may be under max revs at WOT with the current prop- Just my thoughts so dont hold me to it ! ( or over rev the engine !)
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Old 22 February 2010, 09:41   #7
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Read somewhere that for every 1 inch dropped in pitch you can safely assume the RPM will increase by 200rpm at WOT. You really need to get a tacho as this will confirm how the engine is operating, and you can make an informed decision, otherwise it's guess-work.
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Old 22 February 2010, 10:54   #8
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Thanks to all the replies, and apologies for my lack of info for you.

To your confirm thoughts then
1) Hydrofoils wont do much
2) get a tacho for the OB. (any recommendations??)
3) try out a lower pitch prop, possibly 13.5 x 14, or 13.5 x 13

do i have that right??

i need a spare prop, so maybe get one anyway??
and then keep the existing one as spare?
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Old 22 February 2010, 11:33   #9
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I was looking at a Boston Whaler Montauk 17 some time ago, if you look at their website there is a technical specs PDF where they have tested one with a 90hp Merc, I think it did 38kn, the prop was a Laser II 13 1/4 x 20" pitch. Its probably a similar boat to yours , ok so different engine but not vastly.
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Old 22 February 2010, 11:42   #10
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thanks for the reply

i think this is the test you are talking about:
http://www.whaler.com/site_media/Per...EnginePerf.pdf

but this shows a prop with a 20 pitch on it? instead of the advice so far to go the other way, to a 13?
am i right?

if this is correct, and it looks simular in weight etc to my boat, a 13.5 x 20 prop would do it.

this is where i get confused!

help anyone?
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Old 22 February 2010, 14:05   #11
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You need a lower pitch prop for carrying the extra load (divers). As it's very laboured with a 15" pitch, then I'd suggest you drop to a 13" pitch and get a tacho on there to check what RPM you're pulling. You should be able to get a tacho (& a prop) on ebay.
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Old 23 February 2010, 02:28   #12
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ok, so i managed to speak to my mechanic this evening. (He's a great chap, and very good, but difficult to get hold of!!)

he agrees with what has been said, reduce the prop pitch to 13, and that should give me another 500revs. this should then be enough to get her up on the plane, and then she will pick up from there.

did really rate the hydrofoils, but i did not have time to talk about why they won't help??

maybe you know more about this.

he also said i need a tacho. he has an old one knocking about that he will bang on, so i can see what the engine is doing etc

so i will let you know how i get on. maybe a while though, as we are away for a few weeks

thanks for all your help

shame there is no reputation system on this forum, 'cause some of you would be getting greenies !!!
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Old 23 February 2010, 07:24   #13
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Quote:
Originally Posted by markymarkP View Post
did really rate the hydrofoils, but i did not have time to talk about why they won't help??

maybe you know more about this.
They are somewhat of a 'marmite' subject on here . I have them and they transformed my small RIB. Its stern heavy and if you powered up at the wrong point of a wave it would be a little un-nerving to say the least!

But I fitted them purely to cure this as the speed & acceleration were fine ( apart from a second pointing upwards). Once fitted ( it was easy , but un-nerving drilling holes in a new engine !) I lost a knot or two off top speed with it dropping from 32 to about 30 - but in a 4m Rib I could only hit this on totally flat water so was not concerned.

It planes at a lower speed now too - down from 12knts to about 9-10knts. So if you aren't getting alot of bow lift and dont need to plane at lower speeds It sounds like they wont do much for you.

Pete
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Old 23 February 2010, 16:18   #14
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i know its a diffent boat but i have a 12 1/4 x 19 pitch on mine, 5450 rpm at 41.5 knots
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Old 24 February 2010, 11:06   #15
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If it's a 90 do you have power trim? if so, tuck it right under (in) for take -off & bring it back out as you get moving.

I suspect the lower pitch for loaded will do the trick tho'.
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Old 24 February 2010, 19:30   #16
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Quote:
Originally Posted by spartacus View Post
Read somewhere that for every 1 inch dropped in pitch you can safely assume the RPM will increase by 200rpm at WOT
Don't forget that if the boat won't get on the plane this assumption isn't true.
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