Marian
New member
We have recently taken delivery of new 5 Metre Selva Open Sportsboat, with Selva Four Stroke 50hp (a repainted Yamaha F50 Outboard - absolutely identical in every other respect). Everything works great until you get to about 3500 rpm, at which point engine begins to stutter slightly until you get to about 4000 rpm, when engine settles down and runs normally, but will only ever reach 4500 rpm even when at wide open throttle (WOT) yet max rev range is supposed to be 5000 to 6000 rpm at WOT. Will run all day at 4500 rpm without the slightest hint of misfire. The harder/quicker you apply throttle the worse the misfire, in fact if you apply throttle very slowly, through the 3.5 - 4K band the stutter is barely noticeable, but if you "floor it" she bogs down quite badly and you think it might even stall (but never does). Regardless of how you apply throttle once it gets to about 4k it runs very smoothly, but never gets above 4500 rpm which achieves about 19 Knots, with two or three up and about 21 knots if on my own. It is supposed to do about 28 knots at WOT.
Two engineers have looked at engine - both say it is set up correctly. They have apparently checked plugs/ignition and carbs/fuel with their computer diagnostic equipment and both say they cannot find a fault and so think it must be the prop. They have also checked the throttle linkage to ensure the physical action is achieving WOT and replaced fuel tank and lines. The prop is a standard Yamaha prop 11 1/8 x 13 - G. Boat and engine together only displace about 500 Kg's - so with one up and fuel - say max 650 Kg's.
As I would have thought that this is a very light boat, I would have expected a 13" pitched prop to at least achieve something near the max rev range, without any misfire. However, the engineer says they want to change the prop down two sizes.
I don't doubt that if I let them change the prop to a smaller pitch, the revs will go up and the reduced load may even stop the misfire, but I am very concerned that this will merely mask the problem, rather than resolve it. I have done some reasearch and it seems that changing the pitch rarely increases speed, as the boat goes forward a shorter distance for each rev of the prop. As the engine is under warranty - I want them to fix it properly.
I am getting to the point where I feel I should demand a replacement engine, but suspect I would be on a "sticky wicket" if I refused them the opportunity to replace the prop - hence my concern!
For the setup I have described, can anyone please indicate what performance I should expect from the standard prop ie 11 1/8" x 13" and whether it is indeed too large/coarse? Would too large a prop even cause a misfire - as even this sounds unlikley to me?
Any thoughts or advice would be very much appreciated as I need to make a decision on how to progress this.
Cheers
Ian
PS. Since writing above have found performance bulletins on large number of boats on Yamaha's website and if anything these would indicate that this prop would be on the small side - not too big, so it should already be pulling the top end revs with the standard prop!!!
<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by Marian on 18/07/2004 16:31 (server time).</FONT></P>
Two engineers have looked at engine - both say it is set up correctly. They have apparently checked plugs/ignition and carbs/fuel with their computer diagnostic equipment and both say they cannot find a fault and so think it must be the prop. They have also checked the throttle linkage to ensure the physical action is achieving WOT and replaced fuel tank and lines. The prop is a standard Yamaha prop 11 1/8 x 13 - G. Boat and engine together only displace about 500 Kg's - so with one up and fuel - say max 650 Kg's.
As I would have thought that this is a very light boat, I would have expected a 13" pitched prop to at least achieve something near the max rev range, without any misfire. However, the engineer says they want to change the prop down two sizes.
I don't doubt that if I let them change the prop to a smaller pitch, the revs will go up and the reduced load may even stop the misfire, but I am very concerned that this will merely mask the problem, rather than resolve it. I have done some reasearch and it seems that changing the pitch rarely increases speed, as the boat goes forward a shorter distance for each rev of the prop. As the engine is under warranty - I want them to fix it properly.
I am getting to the point where I feel I should demand a replacement engine, but suspect I would be on a "sticky wicket" if I refused them the opportunity to replace the prop - hence my concern!
For the setup I have described, can anyone please indicate what performance I should expect from the standard prop ie 11 1/8" x 13" and whether it is indeed too large/coarse? Would too large a prop even cause a misfire - as even this sounds unlikley to me?
Any thoughts or advice would be very much appreciated as I need to make a decision on how to progress this.
Cheers
Ian
PS. Since writing above have found performance bulletins on large number of boats on Yamaha's website and if anything these would indicate that this prop would be on the small side - not too big, so it should already be pulling the top end revs with the standard prop!!!
<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by Marian on 18/07/2004 16:31 (server time).</FONT></P>
