1971 Donzi Classic 18 power....

[Classic18]

Update: I have the boat in the water now. The motor has splayed mains, lightweight 0.060 Wiseco pistons, line bored, L31 heads with full roller valvetrain, aggressive comp cam, Holley 770 with Edelbrock 7516 air gap intake. Runs very strong. I am currently running 60mph at about 5000 rpm with a 14-3/4 x 21 prop....moving to a 25.

Although I have not been over 60 mph yet (engine break-in), any experience-based suggestions to improve higher speed stability?

[FUJIMO]

Although I have not been over 60 mph yet (engine break-in), any experience-based suggestions to improve higher speed stability?

...Nope. Not on that hull design.

[Greg G]

It is a round bottom 24 degree so I would suggest a 4 blade for general balance all around. You can pick up a Quicksilver Q4 for cheap and it is the same prop as a Rev4. They both run a touch stiff.

[FUJIMO]

I am currently running 60mph at about 5000 rpm with a 14-3/4 x 21 prop....moving to a 25.

Most performance pleasure boat V-8 inboard or sterndive engines maximum rpm is right at 5,000 rpm's, for the best longevity/safety/fuel consumption. Sounds like a 21'' pitch is best. Now you can simply test various types/brands of 21's, to get your best all-around performance.

[Classic18]

Seems like skags towards the stern would help...anyone have experience on mods?

Also, I was told by Mercury Racing that a four blade is generally used for bigger/heavier cruisers, not a light performance boat.

[Greg G]

Seems like skags towards the stern would help...anyone have experience on mods?

Also, I was told by Mercury Racing that a four blade is generally used for bigger/heavier cruisers, not a light performance boat.

What is a "skag toward the stern"? Whoever told you that at Merc does not realize that you do not actually have a light, performance boat. Try a Rev4.

[Classic18]

A slag is a shark fin looking device commonly used under the midship of many inboards. I'm wondering if it would anchor it if placed at the stern

regarding the mercury comment, I told him the boat was an 18 Donzi but maybe he's confused. What do you think the 4-blade will improve? Seems the boat wants to kick right under moderate accel... it's a LH prop

[Davidlake]

The skeg is a bad idea. Driving that hull (boat) just needs seat time to learn how to trim, turn and cope with the chine walking. I have been driving one since 1996 and never been out of control. Just a learning curve. Be careful and learn the idiosyncrasies of the boat. My boat will now go 70+ MPH, but I will only do that by myself! My Skater is approaching 120 MPH. I have taken two high performance driving courses to learn how to drive it... Still will only exceed 105 when by myself. Still learning.

[blacktruck]

Classic18, Any pictures of boat and the engine?

[blacktruck]

What is a "skag toward the stern"? Whoever told you that at Merc does not realize that you do not actually have a light, performance boat. Try a Rev4.

Greg is absolutely right here. That'll probably be your best bet.

[Classic18]

So the thought is the 4-blade will provide more high-speed stability?

[Classic18]

The skeg is a bad idea. Driving that hull (boat) just needs seat time to learn how to trim, turn and cope with the chine walking. I have been driving one since 1996 and never been out of control. Just a learning curve. Be careful and learn the idiosyncrasies of the boat. My boat will now go 70+ MPH, but I will only do that by myself! My Skater is approaching 120 MPH. I have taken two high performance driving courses to learn how to drive it... Still will only exceed 105 when by myself. Still learning.

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ok, that makes sense....but why do you say a slag is a bad idea?. Also, what was the high speed driving no course.. for boats? Where?

[Davidlake]

Skeg is meant for tournament ski boats to keep them tracking straight when a skier is trying to pull the boat off line. These boats are flat bottom at the transom and tend to turn very flat. A "Donzi" hull leans more than most boats in turns. The skeg will be out of the water during turns and touch as waves hit it causing very erratic boat behavior. Sometimes grabbing and trying to straighten the the boat in a leaning turn. Very dangerous! Tres Martin travels during the summer to lakes around the country, providing HP driving courses. A bit $$ but so is your life. Also, I made friends with Shaun Torrente and he came to my lake to school me on the Skater. He is a national and international champ in F-1 boats and has owned a 28 Skater like mine. Both were very good for my needs. Learned to anticipate dangerous situations and correct for them before they bite you!

[Classic18]

That helps...but one correction the skag I'm proposing would be in the stern area under the engine which does not come out of the water.

[Greg G]

That helps...but one correction the skag I'm proposing would be in the stern area under the engine which does not come out of the water.

Don't take this the wrong way but you keep getting solid advice and then you ask again and again why your idea won't work. Many of these folks are not in there first rodeo with your style hull. The skeg idea will be unsafe on a round bottom 24 degree no matter where you put it, not to mention a waste of money on resale. If you want to use up some money extend the inner lifting strakes on that hull right back to the transom.