1997 18 foot lund alaskan porpousing (90merc)

[Lundman83]

Hi folks! Having a problem im hoping someone can help with.
I have a used 1997 18' lund alaskan with a 90 merc outboard.
At 40 mph it starts to act like a dolphin, front end bouncing up and down. If i load the nose super heavy, it will get to 45 mph before it happens.
The only way to stop it is to trim down, slowing the boat.
I installed a hydrofoil, which seems to have helped fuel consumption and handling considerably, however the porpouise is unchanged!!
Any suggestions??

 

[ezmobee]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Smart tabs. Smart Tabs Trim Tabs

 

[TyeeMan]

Re: 1997 18 foot lund alaskan porpousing (90merc)

As you've found out loading the boat properly is going to be half the game. Could be that you're just trimminig up too far. Personally I'd loose the hydrofoil. Do you have a stainless prop, or is your prop cupped at all? If you upgrade to a stainless prop that gives the back of the boat a little more lift it might help you.
Is your Alaskan an open boat with a tiller, , kinda like the WC series Lunds only wider and longer? If so that's a pretty light boat for a 90 hp, even if it is rated for it. Id go more for weight up front. Yes, trim tabs would likely help too.

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Its an aluminum prop, as i live on the praries, and fish fresh water. The prop is undamaged.
It is a console steering wheel type boat. Power trim.
I had approx 500 lbs as far forward in the nose as possible, and just me in the middle at the wheel (240lbs). It only increased my top end 5 mph, before the bouncing started.
I have been lookin at the trim tabs, but they look like they are pretty easy to break no? Is there a stainless option in the trim tabs or just plastic?

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Here is a pic of my boat when i bought it

 

[Silvertip]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Check the position of the AV plate in relation to the bottom of the boat with the engine straight down. The plate should be flush to an inch or so above the keel. If the engine is mounted too far down that creates a long lever action and the thrust from the prop tends to force the bow high but can't hold it up hence the porpoising. Sometimes you simply have to "power through" the issue. I agree that Smart Tabs would likely cure the issue as well. And I too suggest getting rid of the hydrofoil.

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpoising (90merc)

Re: 1997 18 foot lund alaskan porpoising (90merc)

Check the position of the AV plate in relation to the bottom of the boat with the engine straight down. The plate should be flush to an inch or so above the keel. If the engine is mounted too far down that creates a long lever action and the thrust from the prop tends to force the bow high but can't hold it up hence the porpoising. Sometimes you simply have to "power through" the issue. I agree that Smart Tabs would likely cure the issue as well. And I too suggest getting rid of the hydrofoil.

Thanks, ill have to check the height! I cant power through the bounce, it just gets worse if you try to go faster. Wondering why everyone is against the hydrofoil? I did notice a definate improvement in stability at speed and improved fuel economy when i installed it. I am definately going to check into the smart tabs.

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

im at work for 2 weeks and so unable to check anything on the boat untill then , but would changing the prop to one pitch lower help keep the front down a bit, and reduce the tendancy to porpoise? or would doing that kill my abillity to plane out and reach top speed? i realize my rpm will increase, but i have power trim so i should be able to compensate for that.

 

[64osby]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Something else to look at would be if your hull has a hook near the transom. This can cause porpoising.

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Something else to look at would be if your hull has a hook near the transom. This can cause porpoising.

Hmmmmmmmm. a hook you say? im 99% certain there is nothing on the bottom of this boat other than the ribs, but i will certainly check it out! also the hull is in very good condition, hardly a scratch in her as the po did not use the boat very much! perhaps he was scared of the porpoise! lol.

 

[ezmobee]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Hmmmmmmmm. a hook you say? im 99% certain there is nothing on the bottom of this boat other than the ribs, but i will certainly check it out! also the hull is in very good condition, hardly a scratch in her as the po did not use the boat very much! perhaps he was scared of the porpoise! lol.

This is what 64 means by a hook http://img.photobucket.com/albums/v682/46auto/Forum Posts/hook2.jpg The common cause of this on a tin boat would be a too-short trailer causing the stern not to be properly supported.

 

[greenbush future]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Something else to look at would be if your hull has a hook near the transom. This can cause porpoising.

I'd bet your hull is pushed up a fraction right in front of the transom on the bottom side, take a level and lay it on the bottom of the hull, it doesn't take much to cause this symptom (a fraction of an inch!!). Your boat shouldn't need any fins or tabs either, not if it's tuned right.

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Ah!!! Ok i misunderstood "hook"!
When i get home again i will certainly be checking the hull!

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

And of course the height of the anti cav plate in relation to the hull!

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

hi folks! its been a while but here goes.. the motor was about 2.5 inches low. using the lower holes in the mount bracket I moved it up so the anti cav plate is now about .5' above the bottom of the transom. I also removed the whale tail fin. took her out on the water, and the porpouise action is worse!! it starts about 5 mph slower than before.


from the picture and description of "hook", I also saw the definition of "rocker"

im thinking it is more likely that my boat has a rocker problem, as when im up to full speed the hull is not riding ''wet'. only the very back of the boat is in the water. so either I need to add some weight to the nose of the boat to compensate for the 350 LB motor, or I have a Rocker problem!!
Thoughts?
I really don't want to drill a bunch of holes in the transom to mount the trim tabs, but it is starting to seem like a much more viable option!

Is this a common problem with this type of boat? the Alaskan has a flat bottom, not a deep V.

 

[Fed]

Re: 1997 18 foot lund alaskan porpousing (90merc)

Your thinking is right, a hook won't cause it but a rocker will.

 

[batman99]

Re: 1997 18 foot lund alaskan porpousing (90merc)

.

If my boat, I'd:
- Remove the hydrofoil. Never to use again.
- Buy / Install Smart Tabs SX. Install on outside postion of the transom. More outside the better.
- Double check the Prop size for the boat and its load. IMO, the 4-blade gets better grip (like snow / ice tires on a vehicle) and does less up/down bow movements. The 3-blade is better for smoother waters. Pick the best prop size and blades for your average waters.

If wondering, I installed / usage Smart Tab SX on my 17ft boat and love them. My next boat will have them as well. Like we say at the lake, "we don't leave home without them...". LOL!

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

I think i will order the smart tabs.
Just curious, why is everybody against the hydroil? After i took it off i experienced a lot more steering effort and tendancy to pull to the right.

I realize the pull can be counteracted by moving the fin (proper name escapes me)

 

[Lundman83]

Re: 1997 18 foot lund alaskan porpousing (90merc)

I was thinking a 4 blade prop might be better! However the lakes i frequent are more often smooth than rough. Will i need to change the pitch when going to a 4 blade?

 

[batman99]

Re: 1997 18 foot lund alaskan porpousing (90merc)

.

The hydroil fin pushes the motor up. The motor is connected to the transom. The transom is then "up lifted" from a center point. This puts extra force / stress on the transom. Less stress on the transom the better. Besides this, the lift is from a single centre point. Take a pencil, put into a vertical up/down position on the tip of one finger and try to balance it. Notice how the pencil wants to fall / flop from side to side. Hydroil fin does the same "single" lift point behavior to the rear of a boat as well. Thus, the boat wants to fall / flop from side to side as well. And, the boat's steering becomes very unstable in the sharp corners as well.

Trim tabs (like Smart Tabs SX) that are properly sized for the boat's length are installed on the lower "most outside" of the transom. Being 2 points and on the outside, the rear "up lift" is distributed and more even. And, they are secured in the much stronger location - compared to a motor fin. Smart Trim tabs don't care about the motor's tilt or trim position. The rear Trim Tabs lift the rear of the boat evenly as well. Take that same pencil, lay horizontal across 2 x fingers and see how the pencil behaves. Like wings on a plan, the pencil no longer flops from side to side. This change to a horizontal "up lift" position is smooth and stable across entire boat. Especially on the sharp fast corners. And best of all, the "up lift" is on the boat's rear hull (not on the motor or its mounts) - which makes the nose level.

As a suggestion, use Google Search and use "Smart Tabs SX review" search string and read their many review contents. While reading, you'll notice lots of folks are dumping their hydroil fins and replacing with Smart Tabs SX or better Power Tabs from Bennett. re: https://www.google.ca/#gs_rn=16&gs_...61,d.dmg&fp=118b3d7bb754c704&biw=1207&bih=563

For a single source of info, read reviews at: http://www.amazon.com/Nauticus-SX9510-80-Smart-Series-Trim/product-reviews/B001DZ2VVC


Hope this helps as well...