What do you think? Raise the engine a hole. Lower??? Why? - I just think there is a bit more performance left. Specs are included from an old thread on iboats.com Not sure about the top speed though. It was done with a chase boat. I posted MPH but that could have been a mistake and the chase boat was just someone I flagged down in the waterway. Tops with my GPS is 27-30 knts depending on load and condition.
Engine at full trim just below cavitation
Engine at half trim to cavitation.
Engine parallel to hull
Engine all the way down
*Old Thread
Hwsiii - You nailed the mph exactly. Thanks so much for your and others help. Anyone reading this -Hwsiii and the other “prop gurus” know that they are talking about. Post your questions and give the information they are asking for. Pictures help.
Quote:
Hwsiii’s quote:
The 90 HP Yamahas are not loaded with a lot of torque, so it is overpowering your motor and eating torque. When you see the semi cleaver you will understand why I said that is the prop you need, very little surface area and it lifts the stern as well, so a lot of torque is not required. With the semi cleaver 688-45930-02-98 you ought to be running about 38 MPH and have a prop slip between 7 to 11%.
Question - Do you think I should go up one more hole? You said the 688-45930-02-98 likes a lot of height. If you need I can send some pictures. At this point I’m happy with the performace. However, I
With a 17 pitch prop, running 5300 rpm, and a 2.00 ratio theorotical top speed (no slip) would be around 37 kts. Since you say you are running 29-30 kts loaded, that equates to 18-19% slip. You should be in the 10-15% max slip range. From what I see of the prop in the pictures, it doesn’t have much blade area. If your trying to get more speed, I would look for a prop with more blade area and/or slightly larger diameter to reduce blade loading and bring your slip down to where it should be. I don’t think raising the engine another hole is going to net you much.
I prop I would try is a Stainless Mercury Vengence prop.
Below are pics of the first prop. Apollo 13 1/4 x 17. Couldn’t get over 4500 rpms. So, big there wasnt room for a tab. Was told this was “too much prop” for the 2strk 90. The Yami prop I have now was reccommended. I did raise the engine one hole and change the prop at the same time so I don’t know what helped the most. The boat has very little deadrise at the stern. Not sure how the “generally” effects the proper engine height???
My previous engine was a 84’ Johnson 115. Had one of those “fins” attached to the cav plate. Used to turn 5500. Before I repowered the Johnson would only turn 5000. Found out a jug was at 85lbs. Also, did not take a picture of where the cav plate was in relation to the hull.
84’ 18 Privateer
08’ Yamaha 90
There are always greater fish than you have caught, always the lure of greater task and achievement, always the inspiration to seek, to endure, to find.
Hairball - thanks for the response. What do you mean by “blade loading”? I have noticed that once I reach max rpms there is still a lot of throttle movement left. I can goto WOT and will notice a change in engine sound but no change in rpms. I was thinking she “just didn’t have anything left to give”.
ChrisV - thanks for the response. Is there a special sealant that should be used on the engine mount holes? I think me and the big tree in the yard are gonna raise her a hole. Besides the obvious anything I should watch out for when raising this engine. It does have the lift-majigger on the engine.
84’ 18 Privateer
08’ Yamaha 90
There are always greater fish than you have caught, always the lure of greater task and achievement, always the inspiration to seek, to endure, to find.
Blade loading refers to the pressure on the back face of the blades. Basically a boat requires “x” thrust to go “y” speed. The prop works very similar to an airplane wing. High pressure on the back face and low pressure on the front face. It is this difference in pressure that generates thrust. More blade area requires less of a pressure difference to achieve adequate thrust at modest speeds. Props with small blade area work better for faster speeds when water acts more like a solid than a liquid so it is more like a screw going through wood. Too much blade loading (low pressure on the front side) causes cavitation resulting in slip which robs power, fuel economy, and speed.
You may be walking a fine line between trying to push a larger boat for a 90 HP engine while still satisfying the recommended WOT rpm range. Like I said, try some more different props. Maybe even a larger 15 pitch with an aggressive cup that would rev a little higher with less slip to actually yeild more speed.
Raised the engine two holes and did some performance tests. I think I’m good to go. Avg slip of 12.85% from four WOT test runs. No overheat or cavitation issues. Cav plate is clearly visible. Much less spray from around the cav plate.
Might try a 19 pitch if the opportunity is there. Big thanks to everyone for your help.
84’ 18 Privateer
08’ Yamaha 90
There are always greater fish than you have caught, always the lure of greater task and achievement, always the inspiration to seek, to endure, to find.
