Sea Ark 1648mv porpoising after upgrading to 40-HP mercury

[Huffduf41]

Hey guys I recently upgraded my Sea Ark 1648mv from a 25-HP mercury to a 40-HP. I originally had it mounted so the top of the motor "bracket" was resting on the top of the transom but it porpoised really bad. I moved it up two holes and it got better but still can't open her up without porpoising.

I have a full size trolling battery and cranking battery in the very back of the boat, they could be moved forward but it wouldn't be fun. I'd also need a new (larger) compartment up there.

The 40 was propped perfect for my 24' pontoon but now needs a different prop (rpms are too high). I'm not sure what prop it needs or if the correct prop would help the porpoising issue. Basically I'm a mess please help....

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https://i1086.photobucket.com/albums/j447/huffduf41/IMG_20130721_202401_699_zpsbe668518.jpg

https://i1086.photobucket.com/albums/j447/huffduf41/IMG_20130721_201920_234_zpscea19d68.jpg

https://i1086.photobucket.com/albums/j447/huffduf41/IMG_20130721_203139_607_zps6ef74a2e.jpg

https://i1086.photobucket.com/albums/j447/huffduf41/IMG_20130721_202608_781_zps01c8a538.jpg

 

[Attwanl]

If you think it's a wieght distribution issue, throw a few sand bags the front. But if your revving to high, that's a different issue.

 

[cva34]

Can you trim down any more..That will help..But if its Propped for 24 Pont thats your biggest problem.Get it propped right and boat loaded about level and your problem will most likely go away.Sometimes a Whale Tail will help..

 

[tomme boy]

First thing you need to do is get the anti cavitation plate at the bottom of the hull or below it. Then you are going to need to put one of the batteries up front. Then get a 15" pitch prop. You have too much weight on the rear.

Or if you know what you are doing, you can beat a hook into the bottom outside edges that will help. This will lift the rear of the boat and make it stop hopping.

 

[Ranchero50]

What's the angle of attack when you are running at speed? Nose up, nose flat, or? What's the rear bottom of your hull look like? Bowed out or have a hook in it?

Ditto, battery up front at the very least.

You can also try adding a whale tail as it'll move the CG forward by shifting the wetted hull further back.

For troubleshooting:
Sandbags first, then move the battery if the sandbags help, then whale tail or home made trim tabs if it still porpoises.

Expect more of an issue when running into the wind or in chop as air builds up under the bow more at speed.

Been here, fix it.

 

[Huffduf41]

Thanks guys I looked tonight and I had one hole left to trim the motor in so I moved it and it helped a lot. I think the right prop might take care of any remaining issue. I currently have a 11.25" dia 10" pitch prop, I don't have a tach but its definitely over reving. What prop should I go to?

 

[Zum]

I'd try a 15" prop,that should cure most of your problems,that 10" is way to small.

 

[flatboat]

I think once you have it propped right it may go away . get a tiny tack to check rpms. Get rpms in upper end of operating range by changing prop. There may be shops around you that will let you 'try before u buy ' . start with prop shaft parallel to boat bottom , not even with the bottom , this will provide the best in line push . load with about your normal load, go try it, record rpms . about 300 rpms per inch of pitch . if it turns 6500 rpms and the operating range is 5500 to 6000 rpms you would I need to use a prop with 2" more pitch ,that would get it down to 5900 rpms ,just an example . .once you get rpms right . work on load distribution .It will take a while and a couple buxs and time to get it right . Do 1 thing at a time , sometimes a boat will bounce at a particular speed all it needs is to go a tiny bit faster to get past that speed. Just remember ,all that time you are tryin' things you are on the water and not at work !