Boat seems slow need prop recommendation

[Twainman]

Hwsii check this out. I have a 17' Mark Twain that runs 34 at WOT(4800) with a 19 pitch prop and from previous experience seems like it should be a little more. Here is the answers to your questionaire for a prop recommendation.

1. 1975 Mark Twain 170VBR
2. 17'2'' x 84 wide 1750 lbs with motor
3. 140 HP
4. 2 people with 18 gallon of fuel
5. Crusing--Pulling tube
6. 23 degrees deadrise
7. 1974 Mercury 1150 ELPT 115 HP
8. 1.7 gear ratio
9. 4800-5200
10. Not a bass boat--not sure if pad bottom
11. Has Doelfin
12. 13x19p prop--no maker
13. 4800 rpm with 2 people and 18 gal of fuel
14. 1010 feet Lake pool level of lake used all the time
15. Full tune and carb rebuild 2 months ago--no change in speed before tune
16. Since last year
17. Still has full paint with no chips seen
18. Maybe a little more speed 42-44 mph
19. Will get info next time out

The boat handles and sounds good when on plane which occurs around 20 mph. If you have other questions I will get info for you


Thanks

 

[steelespike]

Re: Boat seems slow need prop recommendation

Is the throttle opening all the way?Carbs syncronized?
Try without the foil ;could be good for a couple of mph.
The antivent(cavitation) plate even with the bottom is considered a starting point.Most end up an inch or so above.A 2" increase in height could be good for close to 2 mph.My info indicates 5,000 to 5,500 rpm.A 17" should put you at "about" 5,200. May result in an increase in speed.But surely will improve hole shot.

 

[hwsiii]

Re: Boat seems slow need prop recommendation

Twain, Spike is right about the foil possibly stealing WOT speed. Right now you are showing a prop slip of 33% which is ridiculous for that setup.

Prop Slip

1. We need to check anti ventilation plate height above the keel, but I can almost guarantee you that the motor is too low on the transom, if the tach is reading correctly.We need to remove the doelfin for all of our testing, and if the motor is too low we need to move it up. Get us two pictures of the motor on the transom showing the height of the anti ventilation plate above the keel.

Anti Ventilation Plate

2. We also need to check that the tachometer is reading correctly, try turning the switch on the back of the tach back and forth a few times to clean the contacts off, and then use a hand held tach to verify the tach on the boat.

3. We need to change to a much more aggressive prop to decrease prop slip and thus increase speed at all RPM including WOT. At this point I believe we probably need a 4 blade 16" pitch prop to reach about 42MPH, BUT this may change as we improve the setup on the boat, and we need to check and verify everything else before we buy a prop.


H

 

[Twainman]

Re: Boat seems slow need prop recommendation

Thanks guys! I will post some pics this evening when off work and remove the fin. When I get it in the water next I will post the results I get from tach and GPS.

 

[Twainman]

Re: Boat seems slow need prop recommendation

Well basically the cavition plate is even with the keel and don't see how it could move up with the power trim and transom height as show with pics. See what you think of this setup?

 

[junior1113]

Re: Boat seems slow need prop recommendation

if i just bought. off with the fin, up with the motor, and a 17 pitch 4 blade prop. that boat will ride nice. 5400-5600 would be good wot rpm for that motor. at least take fin off and raise motor rpms should increase.