Engine too low? (pics, specs & Performace) NOT anymore!!!

[mswalter2009]

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 hwsii and others.

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. 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 still have 4" of throttle (at the remote) with no difference in rpm just a slight diff in engine sound. It seems like there is more. I want to get a picture of the cav plate at different rpms. Just can't safely do that by myself.

For anyone reading this post here's the situation -Just repowered my boat with a new engine (specs below). Worked with the dealer on prop size and I went with a less expensive ss prop by michigan wheel. It was the Apollo Series 13 1/4 by 17 SS prop. First test run was 4500 max rpm at WOT. Compared to the yamaha prop listed below this is massive prop. So much so that the yamaha high clearance anode trim tab did not clear the prop. Started searching this site and Hwsiii consulted. Also contacted the company that rigs Privateers. Both said - the prop you need is the yamaha semi cleaver 688-45930-02-98. Had 8 hours on the engine and kudos to the dealer, they traded me back with only the difference. $74.

Todays Test -
The dealer swapped my the appolo prop (13 1/4 x 17) for the yamaha #688-45930-02-98 (13 x 17) Put in after work and the difference was much much improved.

Boat weight - 1250 dry

Boat -1984 Privateer Retriever 18' center console

Sea Level -level, charleston sc

Boat hull - Bay boat, slight deadrise at stern. no pad

Test weight - 1950, includes engine. One person, some gear, Some boat "sodas" 10 gallons of fuel in center line tank. Ya, I'm a big guy.

Engine height -# 2 hole (was on #1 lowest, raised one hole for this test)

Engine -2008 Yamaha 90 TLR 2 stroke

Gear Ratio: 13:26 (2.00)

Prop -Yamaha #688-45930-02-98 (13 x 17) Black SS, semi clever

Fuel - 25:1 (last hour of break in) 86 octaine

Cav plate - could see cav plate above waterline at 4100 to 4800 rpm. A good bit of spray around cav plate but above waterline nevertheless.

WOT rpm -5200 into the wind, slight chop in waterway, 5300 down wind.

Wind - 13 to 15 knots

Speed -38 mph (gps) (this was done with a chase boat) I'm not sure about this anymore. With the wife and kid I only get at tops, with the wind about 28-29 knts. (my GPS)

Engine trim - just below cavitation. bow lifted nicely. Started to porpoise slightly, lowered engine a couple of ticks, still 5200 / 5300 rpm. So, I coud trim a couple of ticks down per the yamaha digital guage and the rpms stayed the same. More that two / three ticks and the rpms would drop a 100 or so.

Throttle response - Can throttle up to 5200/5300, still have about 4" of throttle left to WOT with no difference in speed, slight change in engine sound. I checked the linkage in the driveway and there is about 3" of throttle after the carbs are wide open, and linkage is at stops.

Time to plane - dont' have this but I will
Time to 38 mph - don't have this but I will

 

[hwsiii]

Re: Engine too low? (pics & specs)

Re: Engine too low? (pics & specs)

Walter, your motor is still too low by at least one notch and I really believe you could bring it up two notches. The higher the motor is the more speed you are going to get out of that combination as that prop does like height. I know it is a pain to change motor height, I have had to do the same thing myself with mine, but in the end it is worth it.
I would move it up two notches, and if you get too much ventilation in turns you will need to move it down one notch, but your best speed will be as high as you can get it without ventilation problems.
When you change to the new oil mixture from break-in you will probably gain another 100 RPM after running it a coulpe of hours with the new mix ratio and moving it up will probably gain 150 RPM, so that should be maximum RPM at WOT, maybe a little higher, and great speed.

Cav plate - could see cav plate above waterline at 4100 to 4800 rpm. A good bit of spray around cav plate but above waterline nevertheless.

Besides looking at the anti ventilation plate pictures, which are SUPERB, the above statement is why I think you can move it up two notches instead of one.

This is what it ought to look at 3/4 throttle and higher, you should be able to see the anti ventilation plate clearly.

Please let me know how this works for you as I always want the numbers for my database, right now I have over 160 boats in it, and I always want to know if I responded with a better prop than you had on it before.

Dale, Kenny or some other people on here can help you much more than I can with the link and sync on the throttle, hopefully one of them will stop by and help you with this.


H

 

[kenmyfam]

Re: Engine too low? (pics & specs)

Re: Engine too low? (pics & specs)

Just make certain you are still getting enough cooling water circulating if you raise the motor.

 

[mswalter2009]

Re: Engine too low? (pics & specs)

Re: Engine too low? (pics & specs)

Thanks everyone for the replies. Special thanks to hwsii for your continued help. At this point I think the best plan is to raise the engine two holes. Give it a test run with the same prop. If no caviation, better rpms / speed then assume engine height is right. From there can experiment with different props.

My previous engine was an 84' Johnson 115. I remember that this engine would cavitate during fairly tight turns. I just knew to trim down (from max trim) just a bit when making sharp turns. But, this engine used to run 5500 rpm (no gps) and could noticably tell the diff between say 5100 and 5500. Hard to explain but the boat "ride" would respond. As I said before my current setup just seems to drag after about 3/4 throttle. Gain just a bit more speed, a couple hundred rpm but it almost seems like the bow is dipping, engine dragging. Also, with my current setup I can make much sharper turns without cavitation. The boat has very little deadrise at the stern so I always figured this lends to caviation on turns.

At this point I am ruling out any engine issues. 50 hours on the engine, no problems, 10 micron water / sep filter. Drain carb bowls when the engine sits up. Solid aluminum tank (thanks Privateer from 84'), new fuel lines, idles great, boat planes like a champ...ect

Again, thanks for the responses and looking for more.

 

[mswalter2009]

Re: Engine too low? (pics, specs & Performace) NOT anymore!!!

bump for hwsii

 

[hwsiii]

Re: Engine too low? (pics, specs & Performace) NOT anymore!!!

Walter, I am very very happy, that this prop does what you want and needed, it just needed to run higher in the water column. This particular prop does not produce tremendous amounts of thrust, thus it also doesn't need a tremendous amount of torque, and the extra drag from the lower unit being too deep in the water has a large effect on its top speed.


H