Keel walk after repower

[captcrunch]

Finishing dialing in props on my 1988 38c repower with c9 cats. Seems like motors were flashed to 575hp tune. As we get near wot there’s some major keel walk starting. Anything that I should be looking into before looking at cutting down the keel?

 

[cww]

Finishing dialing in props on my 1988 38c repower with c9 cats. Seems like motors were flashed to 575hp tune. As we get near wot there’s some major keel walk starting. Anything that I should be looking into before looking at cutting down the keel?

As I understand it, the issue is more with the lack of strakes in the Hatteras hull designs of the 1970s-1980's more than the keel. Strakes induce air bubbles into the slipstream, which avoids a suction affect that develops as you go faster, resulting in lost speed and the boat becoming tender and leaning over.

Hatt's are unique for not having these, strakes are present in one form or another on every other fiberglass sportfish I have ever seen out of the water besides Hatts. Many other boats have keels but still do not exhibit this suction behavior that develops as you get closer to 30 knots, so at least to a large degree it can't just be the keel. There are people who have now added strakes/chines to the hull to induce air into the flow and solve this problem, with good results. One of them just posted a thread about this exact thing not long ago on the forum, here:

https://www.samsmarine.com/forums/showthread.php?38922-gambling-experimenting

 

[captcrunch]

Definitely something I’m going to have to look into. We were almost 30 knots wide open and still have to tweak the props to get another 150 turns. Beyond impressed with speed etc

 

[Pascal]

Definitely something I’m going to have to look into. We were almost 30 knots wide open and still have to tweak the props to get another 150 turns. Beyond impressed with speed etc

What matters is cruise speed. Any issue at cruise? If not, how often will you be running on the pins ?

 

[Bob Bradley]

What matters is cruise speed. Any issue at cruise? If not, how often will you be running on the pins ?

Only in an emergency. And you certainly wouldn't want instability in that case if the boat can do the speed, it needs to do it safely

 

[Captain Ralph]

Just like building lots of horse power in our old cars, we forgot some important things; suspension, clutch, gear, brakes and finally, rubber (traction).

So, You have installed lighter engines making more horsepower, pushing faster than original hull design.
And surprise, you may have some problems.

So please tell us what reduction, props and RPM your making before and during your events.
Does trim tabs make it better (probably knot).

 

[Pascal]

Only in an emergency. And you certainly wouldn't want instability in that case if the boat can do the speed, it needs to do it safely

30 years, tens of thousands of miles between cape cod, the gulf and the Bahamas and I can’t remember one time I had to run above 75%

I love the extra HP so the engines have an easier life and last longer. A couple of extra knots on the pins don’t matter.

 

[captcrunch]

1.77-1 reduction wot on the c9 cats are 2500 running a 26x32 3 blade wheel with #7 cup

 

[schuter]

Fast forward to about 22:30 and he talks about keel walk on older Hatts.

https://www.youtube.com/watch?v=dirBIsgblt4

 

[Sonic Boom]

Here's 30 knots in a Detroit powered 46 with lift strakes, stable as a rock. Trim tabs get sensitive at this speed though.

 

[captcrunch]

So it sounds like lifting strakes are most likely the answer. Any idea what it would cost to have done and is there anyone knowledgeable near Long Island ny or am I best getting boat down to the Carolina’s?

 

[Captain Ralph]

Worth review;
https://www.samsmarine.com/forums/showthread.php?38922-gambling-experimenting

 

[Sonic Boom]

So it sounds like lifting strakes are most likely the answer. Any idea what it would cost to have done and is there anyone knowledgeable near Long Island ny or am I best getting boat down to the Carolina’s?

Call Derek at Blue Devil Boatworks in Wanchese, he trimmed the keel and added lifting strakes to a 38 Hatt like yours earlier this year. If memory serves, that particular boat was bow steering but he may have some knowledge to share

 

[rsmith]

Only in an emergency. And you certainly wouldn't want instability in that case if the boat can do the speed, it needs to do it safely

Bob you should understand more than anyone. Fishing the NE canyons every extra minute you can get with lines in the water is valuable. Another 5-6 kts. On a 3-4 hour run can mean 30-40 more minutes of fishing.