Selva Overrevving

Phil Robinson · 27 June 2005, 13:43

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

Pete7 · 27 June 2005, 13:56

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

ADS · 27 June 2005, 13:59

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!

Phil Robinson · 27 June 2005, 14:02

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

Pete7 · 27 June 2005, 14:08

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

Phil Robinson · 27 June 2005, 15:37

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

Pete7 · 27 June 2005, 17:56

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

DM · 27 June 2005, 18:15

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.

DM

Graham H · 27 June 2005, 18:37

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

Phil Robinson · 29 June 2005, 07:37

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

DM · 29 June 2005, 07:58

Engine looks to be to low and with those doelfins at that depth, you'll get handling quirks.

First thing I'd try would be to remove doel fins and raise engine until antivent plate is a touch above the keel.

DM

Pete7 · 29 June 2005, 08:19

Phil, yep agree with the old man, ditch the fins and raise the engine. Also worth following Grahams advice and take a trip to Bill Highman in Worsely. Facinating place (think of car scrap yard only for outboard engines) and get a spare prop. That way you can eliminate the prop as contributing to the problem. When it was rebuilt it might be slightly out of shape adding to your problems. A spare is always good to carry anyway.

Good news is you don't need to cut the transom and with a couple of mates, a case of beer and a day at the seaside "testing" you should be able to solve this one.

Pete

Phil Robinson · 29 June 2005, 19:39

Thanks guys,

I'll give it a go & post the results.

However, I need educating (if you don't mind that is). In my very limited sense of logic, I would imagine that the further away the prop is from the air above it & deeper in the water, the less chance I have of getting air around it. Not that I doubt your wisdom, but I'm lacking insight in the science of props & engine positioning. Also, I don't understand how the plates can reduce the performance.

Someone once said to me that this stuff is a bit of a "black art". I'm starting to believe them! Or can you explode the myth for me?

Again, everyones help & input is very much appreciated.

(PS. I know, I sound thick but - the antivent plate, is that what the doelfins are attached to at the moment?)

ADS · 29 June 2005, 19:46

Quote:

Originally Posted by Phil Robinson

Thanks guys,

I'll give it a go & post the results.

However, I need educating (if you don't mind that is). In my very limited sense of logic, I would imagine that the further away the prop is from the air above it & deeper in the water, the less chance I have of getting air around it. Not that I doubt your wisdom, but I'm lacking insight in the science of props & engine positioning. Also, I don't understand how the plates can reduce the performance.

Someone once said to me that this stuff is a bit of a "black art". I'm starting to believe them! Or can you explode the myth for me?

Again, everyones help & input is very much appreciated.

(PS. I know, I sound thick but - the antivent plate, is that what the doelfins are attached to at the moment?)

Yes the antivent or cavitation plate is what your doelphins are currently attached to. I would unbolt these and try it straight away without them as doelphins do funny things to ribs -they work on some applications but by no means on all!

Pete, would the prop bushing be a possibility or am I way off!!!!

regards

Alex

Pete7 · 29 June 2005, 20:34

Quote:

Originally Posted by ADS

Pete, would the prop bushing be a possibility or am I way off!!!!
regards Alex

My thoughts too, if the prop has been repaired once already did they take the bush out before welding it up....who knows, but changing the prop would eliminate it as the problem.

Pete

Graham H · 29 June 2005, 20:46

[QUOTE=Pete7. Facinating place (think of car scrap yard only for outboard engines)

Hes got draws full of used parts , I could spend days in there.
Graham

Phil Robinson · 18 July 2005, 17:19

All,

Thanks for the many bits of advice. I tried the easiest thing 1st and did a bit of prop switching. I went down to a 15 from a 17 pitch. It worked!!!!
So that saves having to lift the lump up. Fantastic. Thanks to all.

However - I have another problem. The dammned thing won't run right. It stalls after a while on tick over, or when going from neuteral to in gear. I reset the tick over highish but not silly - no change. Starting is a nightmare. Having taken the plugs out, I see they're oiling up really quickly. A road mechanic told me that it can be a common problem in 2 strokes as the 50:1 mix is technically correct but not all motors like it. So I tried diluting the fuel mix a little & it got better. Also, the plumes of smoke reduced in size. Now, I've always been careful with a 50:1 mix, but I have a mate who swears by a 75:1 on his Merc 115 brown band. The mechanic says 75:1 wouldn't cause damage but I'm worried. Does anyone have any opinions?

If I screw this one up, the wife won't let me buy a 3rd engine!

Pete7 · 18 July 2005, 23:14

Okay here goes. Phil you still need to lift the engine on the transom. Not difficult, 3 lads and a set of spanners and 15 minutes work. Don't lift the engine put the skeg on a wooden block and use the trailer to lower the stern of the boat then refit the bolts. Having lifted the engine try her at sea without the fins.

The starting problem could well be in the carburation. How old is the fuel ? lead free has a limited life. Fuel mixed with 2 stroke oil it forms a gum in the float bowls and jets which if left then sets to a varnish. Clean fuel doesn't seem to remove this but you could try a can of carb cleaner. If that doesn't work then the carbs & reeds need cleaning out. Stick with the 50:1 for now.

Do you have a rev counter? I think the 15" prop a little small, please keep a check on the max revs you can pull with that prop.

lc0021 · 19 July 2005, 10:09

100HP on a 5m rib ? - that seems to be a lot of power ?

I'm not an expert tho' - just going by my own set-up - 4.7m Avon with 50HP and goes very nicely - max rating is 70HP....

ADS · 19 July 2005, 10:34

Quote:

Originally Posted by Phil Robinson

Hi all,
I have a Selva S1400 / 100hp (1994) fitted to a narrow beam Osprey 5.5m rib (approx 1988)

its a 5.5m still plenty of ponies though!