Porpoising and trim issues with the new motor - looking for ideas

[Uncivil]

I have a 94 Hewescraft 16' Searunner that I have just repowered. I just put a 75 Hp merc on which is a lot heavier than the original 70 hp Evinrude 2 stroke. So we have noticed a few issues running the unit and I would appreciate some ideas on solving them if possible.

First it seems as if we are sitting down about 2 inches or so at the stern. Not too much but it appears noticeable. One of our thoughts was to move the batteries from the back to the mid Hull position or just under the windshield. I am wondering if anyone has moved teh batteries that far forward and if they have had any problems with pounding? I think the mid hull position would be better but the batteries would be in the way most of the time. What say you folks?

Second issue is porpoising at speed unless the motor trim is set down as far as possible. This causes the motor to be leaning "back about 10 deg. I am wondering if the height of the motor on the transom would affect this. The hull has a rear deadrise of 6 deg and front deadrise of 25 deg. The anti- cavitation plate is set about even with the bottom of the hull. I have room to move the motor up about 1" or down an inch if necessary.

I am also wondering if being down at the back because of the additional weight would impact the porpoising?

Previously we used a "whale tail" on the Evinrude because it came with the boat from the previous owner. I am wondering if adding one will help. Or would any combination of these "solutions" help.

Finally we are planning to go form a 17 inch to a 19 inch pitch to get ou revs down a bit from 5600 at wot to 5000. Any advice will be appreciated.

 

[BonairII]

Re: Porpoising and trim issues with the new motor - looking for ideas

Finally we are planning to go form a 17 inch to a 19 inch pitch to get ou revs down a bit from 5600 at wot to 5000. Any advice will be appreciated.

Do you happen to know what the suggested operating range of your motor is? You want to be at optimal rpm for your motor NOT optimal range for better boat ride.

Couldn't hurt to throw some sandbags in the bow of the boat to see if the ride improves. Much easier than relocating batteries etc...only to find out that it didn't help.

 

[sschefer]

Re: Porpoising and trim issues with the new motor - looking for ideas

Sounds to me like you're propped O.K. the 19 probably would be the wrong solution. Porposing is primarily an indication of insufficient power where the motor tries to lift the bow like it should and gets it there momentarily but can't hold it. That's what the book will tell you but it can also be where the prop type is incorrect and doesn't provide enough stern lift and the bow is lifting when it should be settling down. That's what I think you're experiencing.

You mentioned a whale tale and some will offer Smart Tabs as the solution but I think the better solution for you would be a 4 blade prop like the Stilleto Bay Runner. It will probably give you the stern lift you need to get the boat up on plane and keep the bow attitude correct. Not many boat shops have test props that they loan anymore because the cost is stupidly high but ask around, you may find a shop willing to let you try before you buy. There are also three blade props with different cups and edge detail that are designed for stern lift. There's not a lot of attention to this problem in the Aluminum prop area so you'll probably be looking at going stainless.

 

[Mi duckdown]

Re: Porpoising and trim issues with the new motor - looking for ideas

Were you porposing before motor change?
Porposing is at matter of trim in most cases.
Need RPM and GPS Speed

 

[ericp501]

Re: Porpoising and trim issues with the new motor - looking for ideas

I installed smart tabs last year.. They work great

 

[joe009]

Re: Porpoising and trim issues with the new motor - looking for ideas

a good hydro-foil would help.put one on my boat it did wonders.

 

[sschefer]

Re: Porpoising and trim issues with the new motor - looking for ideas

Were you porposing before motor change?
Porposing is at matter of trim in most cases.
Need RPM and GPS Speed

You are correct, we use trim to overcome power and prop issues and when the are severe and uncorrectable we go with trim tabs and whale tales. In this case I really think a prop change is the first order of business. Think of it like at least getting it in the ball park and then fine tuning it with trim and maybe tabs if necessary.

 

[steelespike]

Re: Porpoising and trim issues with the new motor - looking for ideas

Most likely the boat had porpoising problems so they added the foil.
imo while effective the foil is a bandaid over a poor setup.Can cause poor handling a reduction in speed and an increase in fuel use.
Please read "read this first" At the top of the page and get as much info as possible to us.
Some folks call it a "new motor" because its new to them.What year is the motor,2 or 4 stroke,
What is the rated hp of the boat?What is the present rpm and speed.
Moving the batteries to the mid section should help. by keeping the ends of the boat lighter yet moving weight forward.
No need to install the batteries just move them and maybe run the motor of a small light battery from a car or garden tractor for the test.
You don't need to go crazy with ss props.A 4 blade al prop like the Solas Amita could lift the stern,tame the porpoising, have better holeshot,stay on plane better at reduced throttle.Respond better at the dock,may lose a little speed.

 

[Uncivil]

Re: Porpoising and trim issues with the new motor - looking for ideas

Ok Thanks for the help so far. I guess I need to do some more testing. First thing though is the motor. It is a year old (2011) 4 stroke Merc 75 hp. It was a lease return from a local marina and it had 183 hours on it when purchased. We were able to pick our motor from the crop and selected the one with just a few nicks on the leg.

Max rate Hp of he hull is 90 hp according to the plate. Optimal RPM Range is 5000 - 6000 Rpm

I am going by memory right now but we are hitting 49 kph at about 5600 rpm. To get to that speed without porpoising I need to push the trim right down.

We had a bit of porpoising issues with the previous 70 Hp Evinrude 2 stroke but we had more down trim "left " after setting speed. We were running about 44 kph at 5000 rpm with the evinrude. We did have a bunged up aluminum prop that might have given us some problems but nothing too noticable.

I will say the the hull is not the most streamlined. It almost looks like a displacement hull from some angles but it is solid.

We have a friend that has a 90 Hp merc same year who has a 19 pitch prop he is willing to let us try out and we will look at that a bit later this spring when the weather warms up.

As far as props go any ideas where I can find unbiased information on selecting one ( besides here of course but I want to research as much as I can).