Twin 150's Mercs non/CR on Wellcraft WA... props?

[flotsam35]

I currently have an an 1988 Wellcraft sportsman w/Twin 150 carb Mercs non/CR. I get 5600 rpm's with 15 1/4 X 15p Alum props top speed is 33mph on gps. I usually cruise between 3200 and 3900 rpms (21 to 27mph) Use Bennett tabs and tilt motors for optimum plane. The hull is 18 deg deadrise and I think weighs around 5000 #'s loaded. I have no probs getting on plane even with 4 adults 4 children and beach gear. I do notice 1 prop will break loose around 3750 rpms in certain seas or trimmed to high. I think this boat top speed should be closer to 40mph. that being said....Economy is more important than speed. Any advice or recommendations would be greatly appreciated. thanks

 

[hwsiii]

Re: Twin 150's Mercs non/CR on Wellcraft WA... props?

Flotsam, my numbers show you should be getting closer to 40 as well, maybe even better. I have a feeling besides prop slip you have another problem as well.
I show your motors with a 1.78:1 gear ratio and a maximum 5,600 RPM at WOT, please verify this. Right now I show your prop slip at WOT is 26%, and that is very high for that boat and motor combination. But to really get it into its upper range of speed I need some pictures and much more information. The numbers you gave do not appear to make any sense for your cruising RPM, as your prop slip is less than at WOT and that is not the way it normally works.

Prop Slip

I would like you to lower the motors to where they are exactly parallel with the keel and run a straight edge from the keel to the anti cavitation plate, but make sure it is flat on the keel, so I can see your motor height. Take me about 4 pictures from different angles so I can see how high the plate is from the bottom of the boat. I also would like 4 pictures of the trim tabs up close where they meet the transom and keel.

Anti Vent Plate

If you will do this we can continue to the next step to rectify this loss of speed.


H

 

[flotsam35]

Re: Twin 150's Mercs non/CR on Wellcraft WA... props?

H-
I've seen your outboard diagram on other responses before and I have checked with a straight edge, I get an exact line to bottom of hull. I live in NY so boat is winterized right now. I can take pics as the motors are in 90degree position to the stern. And pics of the tabs as well. I have problems around 3500 and 4000 with the stbd prop will lose grip and rpms will rapidily rise. Could this be a spun hub? I can recover by back throttle and trimming in. I can post pics in a few days thanks for input.

 

[flotsam35]

Re: Twin 150's Mercs non/CR on Wellcraft WA... props?

Oh your other ? was to verify rpm and Ratio. The wot was verified right before I took boat out of water. The cruising rpms was a guess. i feel my optimum rpm was around 3700 doing about 25 mph(approx). I only used the tabs for comfort in certain chop or level out ride. The motors are 1988's that I don't think the ratios were changed. So 1.78 sounds right. I plan on doing impellers in March I can verify then. I have a c/r lower in my shed how do i verify ratio? If i use it to the ratios have to be Identical? Thanks

 

[Dhadley]

Re: Twin 150's Mercs non/CR on Wellcraft WA... props?

Is it the port motor / prop that loses grip?

To get the ratio you'll turn the driveshaft, in forward gear, and count the turns while a friend watches and counts the propshaft turns. When the propshaft turns 10 complete times you'll stop. If the unit is 1.78 like you suspect you'll have turned the driveshaft approximately 17.8 (almost 18) turns. If you've turned the driveshaft about 18 and a half it's a 1.85 or 1.86. If you turned it 20 times it's a 2:1 case.

 

[flotsam35]

Re: Twin 150's Mercs non/CR on Wellcraft WA... props?

Mostly the Stbd side that loses grip. turning the drive shaft and counting the prop shaft revs.....seems too easy thanks for the tip