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Old 27 June 2005, 09:43   #1
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Country: UK - England
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Make: Osprey 18 appx 1988
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Selva Overrevving

Hi all,
I have a Selva S1400 / 100hp (1994) fitted to a narrow beam Osprey 5.5m rib (approx 1988). She used to have a Merc 80 blue band (1969) on her tail. The Selva runs well, & jumps up onto the plane. When I give it some welly, the engine over revs, sounding like I have air around the prop. Drop the speed back down a bit & all goes fine again. Also, in the sea, the engine over revs on waves, coming back to normal in the wave valleys. I used propking & got the right prop & pitch (13.25R17). I also fitted cavatation plates, with no change. I'm now thinking that the shaft is too short for the boat, but I don't know what the length was on the Merc. Selva tell me that they made only 1 length of long shaft as a general non racing item. Long winded, but can anybody give me some advice?
Cheers
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Old 27 June 2005, 09:56   #2
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Phil, lower the engine by one hole on the engine mounting bolts and then take it for a spin. You will need 3 people two to hold the engine when its unbolted and one to lower the trailer jockey wheel which will raise the transom then do up the bolts. 10 minutes work, tops.

You have one of the very first Ospreys btw, model name was "18" which subsequently became the sparrowhawk.

Pete
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Old 27 June 2005, 09:59   #3
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Welcome to the forum! There are lots of very helpful people here some with ribs and some without!
I'm assuming the Selva is a long shaft? Where abouts is the engine cavitation plate in relation to the bottom of your hull?

regards,
Alex


EDIT: Just seen Pete's post, take his advice!
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Old 27 June 2005, 10:02   #4
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Selva overrevving

Pete,

Thanks but I'm already at the lowest point. Any more & I'd have to cut down on the transom. I'll gladly do it, but I don't want to do this & find that I'm cutting away a perfectly good piece of wood, and barking up the wrong tree! It could be an expensive & time consuming mistake. Could there be another problem I've missed?

Phil
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Old 27 June 2005, 10:08   #5
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Arr, hmm. Not sure about the very early Ospreys, later ones had a 21" transom so that a standard long shaft engine at 20" would have the cavitation plate one inch above the bottom of the boat. You might want to do some measuring first, but don't cut anything just yet.

Other option, is the prop stainless steel ? give Julie a ring at Steel Developments and ask her which other manufaturer props fit Selva then off to e bay for a second hand one or ask on here to borrow one.

Did the "cavitation" plates do anything useful ? and is the engine trimmed correctly ?

Pete
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Old 27 June 2005, 11:37   #6
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Selva Overrevving

Pete / Alex,

Your input is very much appreciated. I'm glad I found this forum, very good to hear of like minded people actually prepared to help without trying to sell you something. Most refreshing!

I'm going to take your advice & go make some measurements tomorrow before taking up any more of your time. If you really don't mind looking at it again, I'll post another reply with the various numbers by Wednesday latest.

Again, thanks for your help.

Phil
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Old 27 June 2005, 13:56   #7
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Quote:
Originally Posted by Phil Robinson
prepared to help without trying to sell you something. Most refreshing!
Softly, softly, catchie........

No problem mate, post a pic of the stern if you can.

Pete
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Old 27 June 2005, 14:15   #8
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Quote:
Originally Posted by Phil Robinson
very good to hear of like minded people actually prepared to help without trying to sell you something.
For Gods sake, don't post that you're in the market for a VHF or a second hand RIB or some powerboat training.

If it's an Eagle or Sparrowhawk, they like the anti-vent plate level or half inch above the keel. Try a bit of weight in the back or a prop with a bit more cup before cutting the transom. Also check that the trim catch bar is on the inner holes. The Sparrowhawk can be a bit syrupy unless it's pushed on a bit.

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Old 27 June 2005, 14:37   #9
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Hi Phil
If you want to try a few different props give Bill Higham a visit, City Rd, Worsley, M28 1BD. 0161 790 7678. He has plenty of used props that he will let try before you buy , rearly helpfull family run business.
Graham
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Old 29 June 2005, 03:37   #10
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Selva Overrevving

Pete et al,

I've done some measuring & taken a few photo's (attached). The height of the transon at it's lowest cut away point at the top, to the base of the vee is 50.5cms. The cavatation plates attached to the upper end of the gearbox sit 3cms below the bottom of the vee when the engine is fully tilted down (most effecient running position so far). I have 12cm of downward movement available before the lower engine fixings disappear below floor level.

I've messed about with the trim whilst moving, using the PTT but it only makes things worse if the engine is raised any.

In response to the earlier question, I can't say how effective the cavatation plates were as I had to have the prop rebladed before it could be used again (the last owner bashed it a bit!). That said, both together did show an improvement. Originally it also overrevved trying to get going, which has now been limited to travelling at speed.

Do I need to cut, or am I missing something?
Many thanks

Phil
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