20' w/ 200 HPDI Yami

fdcityronco · #1 01-14-2006, 12:13 PM

New to web site. I have a 1970 20' w/ Yamaha 200 HPDI mounted on a bracket with bobs manual jack plate, The performance is great, what a difference from the old 89' Johnson 150 GT. Little heavy in the rear though, I might need to add more flotation, any thoughts? The bottom of the platform sits about 2 inches above the water line. Performance was 51 mph @ 5400 rpm. I'll post some pics as soon as I can.

strick · #2 01-15-2006, 12:58 AM

when you say "add more flotation" what exactly do you have in mind?

strick

fdcityronco · #3 01-15-2006, 01:18 PM

I was thinking of having a boxes shaped just like the one for the main flotation, just a little smaller, welded and attach them one on each side of the main flotation. I did an experiment and took two empty 4 gal containers and stuck one on each side under the bracket two see if it would raise the rear and it did. about 1.5 to 2 inches.

strick · #4 01-16-2006, 01:41 AM

That would be a good idea. I was just making sure that you were not thinking of adding foam to your boat like some guys think will add flotation. Some guys have added boxes on the transom of their un bracketed 20's to increase flotation. I have a pic somewere if I can find it I'll post it.

strick

gf · #5 01-20-2006, 10:30 AM

Quote:

Performance was 51 mph @ 5400 rpm.

From your RPMs it sounds like you are propped right, but I would think you could get well over 50 MPH with a 200 HPDI. Your engine is probably too low in the water.

My McKee Craft 196 is a little bigger than your 20 and I can hit 48 MPH with the 150 HPDI. The 196 with a 200 HPDI will go 60 MPH.

fdcityronco · #6 01-20-2006, 10:28 PM

Ya, I think you might be right w/ the motor being a little to low I'm still tinkering with it in between work. The biggest problem I'm having now is a little bow hop when on plane. I wonder if it will help moving the motor a little higher up.

Blue_Heron · #7 01-23-2006, 02:30 PM

Hi,
Bow hop, or Porpoising, is all about too much weight in the stern (or not enough weight in the bow). As noted in previous posts, it can be fixed in a number of ways.
* Trim Tabs
* Thrust angle
* Weight shifting
I have a 18' Action Craft with a Merc 150XR4 on a hydraulic jack plate. With that big motor hanging cantilevered off the transom, it used to porpoise bad at low speeds. I have trim tabs (Bennett), but didn't like the idea of dragging them through the water to keep the bow down. I added transom wedges and now I can stop the porpoising by trimming the motor down.

Unless you're going to add trim tabs anyway, I'd start with transom wedges or set up the jackplate for more down-trim on the motor. If you add flotation to your bracket, it will help with your displacement trim (at rest or off plane), but may actually hurt your planing trim unless the addition increases the surface area of the hull bottom. Once the boat is on plane, it's just more weight hanging off the back of the hull.
Dave

P.S. Why is a guy with a flats skiff posting to ClassicSeacraft? Last week, I added a 20' CC Seacraft with a 200 Merc to the fleet. Spent part of the weekend putting trim tabs on her. All I can say is God bless the woman I married. [img]/images/graemlins/smile.gif[/img]

eggsuckindog · #8 05-15-2006, 04:36 PM

Yes raising the motor to the proper height will help, maybe not eliminate but will help, it did mine.

sailcat · #9 05-16-2006, 02:07 PM

B-H, I to have a 200 on a 20'. Mine gets a little light in the bow above 40 mph. I hate adding the drag using my tabs, so am going to raise and add wedges to the motor.
Am also wondering why a flats boat is trying to compare #'s with a Seacraft hull. With my T-top acting as a parachute, mine does around 53mph @ 5500.