EricM's progress report/ ideas needed

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EricM

Well-known member
Joined
Jan 24, 2007
Messages
8,593
Location
Harrison, TN
As had been previously reported, I changed engines on my 19-1/2 ft long, 8 ft wide semi-vee tunnel hull center console bay boat. I replaced the Honda 90 with a Johnson 150.

The 21" prop I had was too much, (4000 rpm, 30 mph, porpoise with much up trim at all) so I dropped to a 17" stainless, and attached a hydrofoil. Tried it out today with the following results:

IMMEDIATELY jumps on plane.
Hits 30 mph in seconds with full down trim.
Slight up trim gives 35 mph and 4400 rpm, but porpoise starts here.
More up trim runs rpm quickly up to 4800, and likely considerably higher, but porpoise beats the crap out of the boat (and me) before I can actually approach the max possible rpm.
Motor can be lowered 2" yet.
No jack plate. Would one help eliminate the porpoise? If so, how big would you think it would need to be?
Will an even lower pitch prop help?

Suggestions (about the boat!!) are welcome. She's totally fishable as is, but I'd like to adjust the wrinkles out of the set-up and make her effecient, solid, and faster.


Thanks, my friends!!!! Eric
 
I have been doing some net research about your problem. There are a lot of suggestions including those you mentioned. The one that I know nothing about but seems to be the best answer is smart tabs or trim tabs. Many people with your type of boat have this problem. There seems to be a transom design problem. This means that you have to lower the motor to get on plane and keep it there to maintain control of the the boat but you will never get max speed and cornering control with the boat at this trim level. Please do some research on trim tabs and see if it answers some of the problems you are having.
Good luck and Merry Christmas to you and the Mrs.
 
Knowing your boat, the jack plate to lower it more will not work because you cant turn the motor enough before hitting the walls of the transom area. A major remodel to accomodate the turning issue might be in play if you can get it lower to see if it will work. Past that, I really cant think of what will work other than possible a long-shank motor.
 
I was thinking more of how the set-back of a jack plate might change the leverage, not lower the motor farther than I can now. There is no problem with prop slippage now like with the other motor or with the 21" prop without the hydrofoil. I'm going to contact the boat manufacturer again, maybe they can help too. Thanks!!
 
old man101 - 12/22/2010 6:26 PM

eric i have and old 8 inch one if you decide you need one.

Thanks!! I'll let you know. It may just do the trick!!!

I did send an e-mail to the manufacturer. The boat is 12 years old and I imagine they have solved lots of issues over the years. Maybe mine is not particularly unique. It's just that to me there is a little black magic getting a set-up figured out!!!!!
 
The more I learn the worse it gets!!! Different prop geometries for hole shot, speed, cruise effeciancy, bow lift, or stern lift - actually a myriad of combinations of all of them using diameter, pitch, and rake angle; blade width and tip and trailing edge cupping; running submerged or half exposed; 3 blade or 4 (or even 5); thru-the-hub exhaust, over-the-hub, or with various size holes in front of the blade to bleed exhaust for the hole shot. I know I have missed a bunch of other prop design factors. Then there is motor height and set-back, hydrofoil design (if used or needed), boat loading (center of gravity), trim tabs, etc., etc., etc.

I will be sacrificing the goat and chickens as soon as the witch doctor arrives....................

Huh - talk about having it easy - Santa only has to get reindeer to fly!
 
Eric, WE NEED TO LOWER THE ENGINE BEFORE YOU GO ANY FURTHER!!!! The way the engine is running now through the water, the "wale tail" is not in the water thus it is not doing it's job! It act a little like trim tabs, but not exactly. Out of the water it does nothing. You need to get it over here and lower it. If it still is out of the water, then we may have to add a plate to lower the engine a little more. If it continues to porpose badly with no trim, I have some trim wedges that give it more negative trim. For some reason, I see more boats with those x-flow Johnsons with them installed. I don't know if it is design or what. Are you pleased with the hole shot though??? The wedges will also decrease hole shot time. emoEek
 
I'm with wrechin2, I've had to run whale tails on previous walleye style boats and it shouldn't porpoise like that if that thing is down in the water, sounds to me like your too high. I don't think you can go wrong with a jackplate either, makes for easy adjustment to figure out the "sweet spot".
 
The first time we took it out, the cavitation plate was running about 2-3 inches above the waterline. With the cowling level with the water idling, the cavatation plate was only 2-3 inches under water. That boat is defanately a strange one!
 
try some weight in the front in, i would start with 50 lbs amd go down ive seen this work well before and actually increase overall performance !! AND YES THE MOTOR COULD BE TO HIGH !!
 
SPEED FREAK - 12/30/2010 12:48 AM try some weight in the front in, i would start with 50 lbs amd go down ive seen this work well before and actually increase overall performance !! AND YES THE MOTOR COULD BE TO HIGH !!
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Knowing how much crap he carries on the boat, weight-forward is not an issue for sure. </p>

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I'm too much of a multi-species fisherman! Since I never know what I'm going to fish for (other than catfish) I tend to carry EVERYTHING that I might POSSIBLY need. However, I do tend to take on water over the gunnel with just a slight chop..............

I was warned that I should have bought a tugboat and a barge!!! emoTongue emoDoh
 


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