Here is a intresting issue

[glassman]

A friend of mine just repowered his kencraft he went from a 150 yam.hpdi to a 150 yam 4 stroke. The boat would run 43 with the aged hpdi now with his 2007 4 stroke he is only getting 37 that is 6 miles per hour. Both are turning max rpm. The 4 stroke burns alot less fuel than the 2 stroke did but it does not perform as well. The dealer tried moving the motor up some holes and down some holes no change, even different manufactures of props no change.

question 1:
Is it the horse power to weight ratio? The HPDI does not weigh as much as the 4 stroke.

question 2:
Is it the way the motor is rated Hp at the prop or at the power head?

question 3:
Are the new emission standards effecting performance?

question 4:
Or does it just come down to the old saying to generate horse power you have to burn fuel?

[cdail28590]

I would say its the weight of the 4 stroke and the fact that I don't think the 4 strokes have the power of the 2 strokes.

[Bullshipper]

To big a difference.

I would check to see if the gear ratio is lower on the output of the $S as compared to the 2 stroke.

If the motor gets max rpm, it probably needs more prop, as is evidence with less fuel burn.

Sounds like the 4 s prop needs more pitch.

[glen e]

yeah, I agree...get ahold of ken at propgods.com for his advice - the only diff you should be seeing is maybe 1 -2 mph diff at WOT and more holehsot out the hpdi...

the f150 actaully makes more hp than the hpdi , I beleive (163 vs 150 something)

[glassman]

I talked to my friend with the kencraft he talked to a number of different yamaha certified mech. They told him the top end loss was excessive but not unusual most of the boats that go from a 2 stroke to a 4 stroke do loose some top end due to the weight and the horse power curve. About the prop he was told that if he is reaching max rpm but not exceeding it than it is propped correctly. He said in a pissed off voice i paid $12,000 total for the motor to be quiet get a little bit better fuel economy and go slower. Then he said i should have bought another HPDI and laughed.

[glen e]

"About the prop he was told that if he is reaching max rpm but not exceeding it than it is propped correctly"

I still disagree...you want to be in the TOP HALF of the WOT range, not AT the max of the range.... - that means he can go up in pitch, gain speed and still be propped correctly...call ken at propgods.com

[offshore]

The kencfraft hull is a great hull but heavy and not very economical. I would be surprised if he will get much more speed out of that combination even by switching props. I ran into the same thing when I went from a 2 stroke to a 4 stroke on my last boat. I lost 6mph on the top end and tried 5 different props but only gained another 1.5 miles a hour on the top end but the mid range suffered. Let us know what happens if he tries different props........Mark

[Kern O]

The increased weight may require tabs to bring the stern up a bit. The change in mileage is great, but I can understand the concern over the loss of top end.

[surfncnow]

Much rather have gas mileage over top end>!>?~~~~

I remember my yz60cc throwing dirt in face of xr80>?

am I off on technologies>>?

[surfncnow]

does croom still exists in mid Fla?