Faulty fuel connector - causing abnormal performance?

[frankgrimes]

Hi Guys,

Been trying a few difference props on my boat(60 Merc 4st), and with my original 13p(Merc) was hitting rev limter(But getting quite good performance - great hole shot, top end of ~31knots), tried a 15p and was getting ~5700/5800 hole shot fairly poor, top end 28/29 knots - these results were with a "faulty" fuel connector(See my thread on Seaway scare)...anyways, replaced fuel connector(All good now!), ordered a Solas prop, and they recommended a 13p, 11.6(Bigger than my merc 13p) - Well with this prop(tested this morning) Im only able to get ~5400, top end 28/29knots(Hole shot not that good).

Are my previous tests tainted (potentially) due to the faulty fuel connecter @ engine? (i.e. It was potentially causing some excess air)...or is the new Solas prop simply not suitable?

Cheers,
Mick

[peterbo3]

Mick, if the diameter of the new prop is greater, then RPMs will drop. I would go up in pitch to a 14 but with the same diameter as the Merc prop.
Or just stick with the original 13" pitch Merc. You are only going to get a small increase in performance for quite a few $$$.

[frankgrimes]

Cheers Peter - Goal was to get a prop that dropped rpm slightly(to avoid hitting rev limiter)...ill get solas to send me smaller diameter 13p...see how that goes, or try same diam. but in 14 as you have suggested.

Mick

[FNQCairns]

I cannot speak of the fuel connector, less fuel but metered properly means you get HP gains in 2 strokes, less again means it either doesn't work properly or you get towed home and the engine gets a fresh rebuild....your testing reads as flawed somehow, the 15 should be the best based on what's there and my crystal ball, the last solas reads to me like it's got too much diameter and that's just a silly waste of time and money on your typical boat used in typical environs....but good for pulling 2 skiers and certain sized stumps.

[frankgrimes]

Thanks FNQ - So the 15 *should* have performed better(but didn't for whatever reason)...so I should try the Solas in smaller diam. in 13p and if no good or try 14p in merc?

[FNQCairns]

Yeah I would make a b-line straight toward testing the merc 14, that last solas reads like a total dog diameter wise when one compares it to the merc 13 in opening post or something else is going on.

Still it will all come down to the propellers diameter/surface area, if the 14 is by ratio large then it will be less useful.

Lower diameter and low cup too = higher pitch possible = higher cruise speeds for rpm = better fuel economy and higher WOT speeds for rpm....but you can go too far although possibly not by much not with today's typical of the shelf examples.

Most often I found 14p props have stupid huge dimaters this also goes for the 13p props....if you can find a 15p with low diameter (closest to 13d and under no circumstance higher than 13.5 for any propeller fitted) then on paper it might suit better than the merc.

what sort of boat is it?

[frankgrimes]

Cheers FNQ - Its a Polycraft 4.5 CC

[Spaniard_King]

Mick,

The fuel connector would not have hampered your testing as your engine is efi and has a vapor seperator. With EFI if they have no fuel you will either surge violently or not run at all so its cut and dried with EFI engines.

We will have to find a 14 for you to try. The 13 is fine to use just dont hold the throttle down

[frankgrimes]

Cheers Garry - Solas were happy to exchange if this one didn't work out...Ill give them a call tomorow, and see what they have available.

[frankgrimes]

Just an update on this - Solas recommended I try a 11.8x11P(I had my doubts!), finally got a chance to test it, and was very suprised with results.

Hitting 6000rpm, hole shot really good, and top end is ~31knots....I tested it yesterday, and it was blowing 25knts, so not ideal conditions to test wot.

Anyone with a Polycraft(4.5) who is struggling with prop selection should try one of these.

Mick

[fisho64]

Hitting 6000rpm, hole shot really good, and top end is ~31knots....
Mick

just curious, isnt that exactly the same as what you had with the original merc prop?

[frankgrimes]

just curious, isnt that exactly the same as what you had with the original merc prop?

Hole shot a bit better with the Solas(Same top end), but Im not hitting rev limiter as I was with Merc

[FNQCairns]

Ok so where you where trying to fit the big foot sized props? sorta kinda for the weight of boat?

Has that new prop got mega cup? On paper i don't like it, 11.8p is not enough travel per revolution to get the best from your package in theory...will do the sums now, your engine is the 1.83 ratio...if so then your quoted max speed is pretty close possibly a little high, how does that prop hang on in turns and at cruise over low rolling/ down the face of bigger swells etc.

because of the low diameter, Check the cup on that prop if it has heaps this will allow enough resistance to run such a low pitch without overreving.

Still it's all about you, the engines conditions have been met and you like the way it behaves so all is good on that front...personally i would be looking for a prop that has a bit more pitch but if this prop doesn't have heaps of cup to create turning resistance you will not get away with a prop of larger pitch but worth to investigate albeit less urgently now.

EDIT: I stuck to convention when reading the pitch and diameter so got it reversed....no of the above counts now..ignore.

[FNQCairns]

Hey how are you measuring your speed? in theory you will not see over 27kn with that prop, what the manufacturer states on the prop and what it actually measures doesnt meed to be correct either so as usual it's not an impossible speed.

[frankgrimes]

Hey how are you measuring your speed? in theory you will not see over 27kn with that prop, what the manufacturer states on the prop and what it actually measures doesnt meed to be correct either so as usual it's not an impossible speed.

Speed measured via GPS