Is my prop right

[Blaster Bretty]

I have a 40hp johnno, eleccy trim + tilt + start + tiller steer on a 12 foot glass runabout, A wee bit overkill but have thrashed it up + down the logan for a good year now with no problem's, My issue is have I got the right prop on the motor, she punche's out of the hole like no other and hits full rpm and top speed in about 4-5second's, great hole shot grunt but top end is only 51.1 Kph or 27 knot's, other blokes rekon this wee boat should really hoot along, and it doe's but given the power to weight + size ratio I feel there should be a bit more top end towards 60Km at least,
The prop is a 12 1/4 X 15, is this a big pushy pushy prop , max RPM WOT is 6200 and trimmed out a tad she hits 6500 (51.1Kph), I have a hunch that the tacho is out a wee bit coz she idles at about 1200 which sounds too high but you can bet that the motor isnt doing that as the idle is quite low. I tried adjusting the pin on the back of the tacho to no avail but should I try a bigger pitch smaller diameter prop to gain more top end, I really dont care about giving up the hole shot grunt as it can be quite scary somtimes at the mo the way the boat fly's up if I give her a fistfull off the line so to calm this by losing the lower end will not hurt. at the moment the boats engine height seems ok, cav plate a tad above the keel line and the boat plane's nice and flat and the trim works as it should so im pretty sure the height is not the issue.
Anyone got any thoughts on this thank's?

Bretty

[FNQCairns]

If you consider a 17p you should do 55km/h at 5500rpm, that is if it can push the 17 to 5500rpm....if you can push an 18 then 58km/h..I assume 2.42 gear ratio.

I don't think you strictly need the extra speed:o and a 2 stroke reving as easily as yours easily is what typically makes a long lasting engine (if you don't use it all very often)but the extra quietness and fuel economy at all typical speeds would make it worthwhile trying some different props.

cheers fnq

[Blaster Bretty]

If you consider a 17p you should do 55km/h at 5500rpm, that is if it can push the 17 to 5500rpm....if you can push an 18 then 58km/h..I assume 2.42 gear ratio.

I don't think you strictly need the extra speed:o, but the extra quietness and fuel economy at all typical speeds would make it worthwhile trying some different props.

cheers fnq

Yeah mate I dont think there will be room for the extra diameter, there is very little clearance between the cav plate and prop tip's,this is why I asked about the increased pitch and smaller diameter, the motor is a big twin cylinder 2 stroke (750cc 2000 model) so they were known for there fair share of vibration's and extra noise etc, in fact im pretty sure these motors were a detuned 50Hp block, it's only the carbie's that were a bit smaller or jetted differently.

Bretty

[FNQCairns]

Yeah mate I dont think there will be room for the extra diameter, there is very little clearance between the cav plate and prop tip's,this is why I asked about the increased pitch and smaller diameter, the motor is a big twin cylinder 2 stroke (750cc 2000 model) so they were known for there fair share of vibration's and extra noise etc, in fact im pretty sure these motors were a detuned 50Hp block, it's only the carbie's that were a bit smaller or jetted differently.

Bretty

Shouldn't be a problem, keep the diameter the same or to one that still fits even if a little larger/smaller than now and search out just the increased pitch portion of the prop spec. I have no definite idea what is on offer around this diameter but I suspect they should be available, testing will tell all.

cheers fnq

[cormorant]

First of all you have to confirm what revs you are doing.
then check what WOT revs should be
Check engine height- go as high as it will take with out airation and not too much cavitation , airation on turns
Get existing prop scanned and chedked for bends etc
If all that is good
if you are already at the max then a different prop design may give more speed
Go for thinner pladed stainless viper style with more rake would be my guess and might get you some more pitch and top end. They will run a little higher in the water so you may be able to raise the motor a hole. Loading of the baot and trim is also critical in a small boat.

If you know all that just ignore this post.

Pointless exercise untill you know 100% your revs

Forgetting top speed it laso has to be uselfull at your normal load in the midrange.

Cheapest way to get a bit extra will be to have the prop shop pitch it up a inch rather than buy another prop.

[FNQCairns]

True the tacho is an outlier, I too have little faith in it but have no way to test it individually although if the gear ratio is known and the prop pitch + a little experience to denote a competent slip factor for the hull at speed it becomes clearer, working back he is ATM doing approx 54km/h at 6200 and 57 at 6500, the rpm is theoretical including slip.

In this instance keeping a little extra leg in the water may not be a bad idea.

cheers fnq

[Blaster Bretty]

Thanks guy's, this was the exact info I was after, FNQ... true ,in reality I dont need the extra speed, she go's great as it is, was more curious if this motor + boat package was performing as it should, after all it is an older glass boat, much heavier than a nice modern tinnie, I honestly thought that I was doing at least 60Kph but once I bought the GPS I was a bit shocked to see only 50Kph coming up, then again this is plenty fast enough, it is only 40 Hp after all. not 200!.... but still if theres a will theres a way...right?
So I think the answer so far is a prop the same size but with more rake?
Will try and post a couple of pics tomorrow, a bit late now, working very early, 4.15am wake up "sheesh"

Bretty

[FNQCairns]

Thanks guy's, this was the exact info I was after, FNQ... true ,in reality I dont need the extra speed, she go's great as it is, was more curious if this motor + boat package was performing as it should, after all it is an older glass boat, much heavier than a nice modern tinnie, I honestly thought that I was doing at least 60Kph but once I bought the GPS I was a bit shocked to see only 50Kph coming up, then again this is plenty fast enough, it is only 40 Hp after all. not 200!.... but still if theres a will theres a way...right?
So I think the answer so far is a prop the same size but with more rake?
Will try and post a couple of pics tomorrow, a bit late now, working very early, 4.15am wake up "sheesh"

Bretty

Choose whatever style of prop rocks your boat or fits best, then if the GPS says 55km/h on a 17p prop you are doing 5500rpm (i think this your marketed upper rpm) or somewhere close to, you can in theory and often in practice ignore the tacho....esp in your instance where it seems untrustworthy.

All speeds hinge on the assumption of gear ratio being correct, I think it is, but if not, garbage in = garbage out, so obviously check this.

cheers fnq