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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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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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