Re: Tohatsu 50HP Prop advice needed...
First of all thanks guys for your replies!
I suspect that your speed is a little optimistic because your slip would only be 2% not impossible but unlikely.
Using a little more reasonable slip of 10% your speed with the 13 would be about 35.9 @ 6000rpm.
A 1" pitch increase would drop the rpm about 150 to 200 rpm So we'll call your 15"pitch rpm 5600 with 10% slip
your speed would be about 38.6.
By the way my limited info indicates your motor is rated at 5700. So your over revving a bit.And a 15 should produce a better wot rpm.
Actually my GPS which I have in mph 99% of the time said 39mph but when I put it in km/h it was 62km/h = 38.525mph, thought I would say as you spent time working out my slip!
RPM range is 5150-5850. Gear ratio is 1.85.
You can throttle back, put the 15 pitch prop on there, or take another person with to pull the rpm down a little with the 13 pitch. Running it over the maximum recommended rpm will shorten the life of the motor.
As far as speed goes; the 13 @ 6000 rpm equals a theoretical 40 mph; while the 15 pitch at 5600 rpm equals a theoretical 43 mph.
Solas offers a SS New Saturn 15 pitch prop for your motor, for even more performance. You'll be able to run the motor height a hole higher with the SS prop, gaining even more performance. That switch(and set up change) could result in as much as 5-6 mph increase.
Is it one of these?
View attachment 195281
No thats not the my boat!
As far as speed goes; the 13 @ 6000 rpm equals a theoretical 40 mph; while the 15 pitch at 5600 rpm equals a theoretical 43 mph.
This seems to confirm about 3 mph difference using a real life 10% slip.
So it would appear you both think I could gain about 3mph by changing from an 11.1 x 13 prop to an 11 x 15 prop.
I should have really given you guys more info from my original post, so here goes...
From memory with my dad in the boat (+ 85kg) the top speed is 35mph and I think the WOT is slightly less, maybe 5800 rpm.
RPM... As stated it would appear I am over revving at 6000 rpm on my own in the boat at WOT. My manual says my Tohatsu 50hp TLDI WOT revs should be between 5150 - 5850 rpm. Though I must say I do not trust my OEM Tachometer!!! I know it is not accurate as when I am on idle, 700rpm (though by pushing the engine key in for a second I can raise it to 800 rpm and pushing it once again raises it to 900 rpm and if you push it again it goes back down, 800 and then to 700, and then back up...) but my tachometer from memory shows the idle as only 500 rpm (when I know its really 700 rpm as controlled by the ECU) so this would indicate my tachometer under reads very low down in the rev range, and the fact it indicates 6000 rpm (from memory!) at WOT when I'm on my own in the boat would indicate it is over reading because the engine/ecu has a rev limiter, from memory at 6150 rpm, which has not kicked in before and I have not received any fault codes showing over revving has occurred, assuming it would?!
Or maybe my tachometer is reading fairly accurately top end when indicating 6k rpm, and maybe by fitting an 11 x 15 prop it would indicate about 5800 rpm at WOT with just me in the boat, and say about 5600 rpm with me and my dad in the boat, either way that brings me into the 5150 - 5850 rpm range at WOT like the manual suggests/recommends!
Also I just noticed on the Tohatsu chart is also states the stock prop 11.1 x 13 is for a boat with a moderate weight load (500-1500 lb.) but it says for a light weight load (100-500 lb.) then a 11.1 x 14 or 11 x 15 prop is recommended. Well according to this I only ever have a light weight load, with just me and my dad!! I now appreciate its very hard to guess and that every boat is different, but with their loading advice in mind and the fact my rib is not a heavy rib (116kg bare boat + 95kg engine + weight of battery/consol/steering/fuel etc I would conservatively guess absolute MAX of 300kg all in) so I would have therefore thought an 11 x 15 prop would be ideal for my rib?! Maybe my tachometer is showing a true 6000rpm and the fact my rib is not that heavy it has the power to spin the 11.1 x 13 prop at that rpm, hence a bigger pitched prop would be ideal to lower the rpm?!
I don't know! Sorry to go on!! I know its not ideal my tachometer under reads at idle so that puts doubts into its accuracy, but to be honest I don't want to and won't go down the route of changing the tachometer, its the OEM Tohatsu one that came with the engine, and my complete package was installed at the Tohatsu UK headquarters 11 months ago, so its not like I installed it! lol.
So going back to the original question, you guys think I could potentially achieve a 3mph increase in top speed (8% increase), well to be perfectly honest I feel a bit begrudged to spend ?150 for a brand new OEM 11 x 15 prop or about ?115 for a brand new 11 x 15 Solas Amita 3 propeller for that increase, BUT I have been looking around and I have found an unused in water second hand 13 spline 11 x 15 prop which I can get for ?30 (it looks in very good condition) BUT I am a little cautious whether this will fit. Basically the seller bought it a year ago from a guy who never used it, it just sat in his garage. The seller bought it for his Suzuki DT50 which is a 13 spline but it would not fit properly. Unfortunately it doesn't come with a box so we don't know the make and there is no part number stamp on it, just the size! He said...
"It does fit along the splines OK, but just doesn?t sit properly when in place and feels as though it rubs against the base of the prop shaft when I turn it by hand, possibly just needs a spacer but I?ve two other props that fit and work OK so I?m loather to fiddle more. There?s no problem with it, it just doesn?t fit a Suzuki, but I do understand your concern..."
Here's some pics...
http://i24.photobucket.com/albums/c24/__james_/001.jpg
http://i24.photobucket.com/albums/c24/__james_/002.jpg
http://i24.photobucket.com/albums/c24/__james_/004.jpg
What are your guys thoughts? Should I risk it for a biscuit?! Unfortunately my boat is in Spain, so it is not a simple task to check whether it fits, though I am next out there in 3 weeks, but it is the hassle of taking it there in my hand luggage on the plane as well as the money if it doesn't fit! Obviously if it fits and works I will be very happy for ?30!
I did a google search to see if Suzuki props would fit my outboard, but no joy, though I found these two threads, which indicate Suzuki prop housing/gear cases are a slightly different size...
post # 6 here....
How interchagable are props? - RIBnet Forums
post # 7 here...
http://forums.iboats.com/prop-quest...-13-spline-prop-fit-omc-13-spline-589766.html
If you guys think from what I have told/shown you that this unknown make 11 x 15 prop will fit my outboard then I will probably give it a try.
Once again, sorry to go on and thanks for your time/help!
James
