Boat Porpoising

[Sitkaspruce]

On Sat, when I went out to wet the lines, the boat would porpoise when ever I reached anything above 3300 rpm. Last time I had it out, it did not do this. The only difference was that I filled up the boat. It was about a 1/4 empty. I tried to move up all the weight I could into the bow, but it changed very little. I have researched a few different options, including going to the boat tomorow and ensuring that the Cav plate is equal to the bottom of the boat. Also adding a "Whale tail, Sting Ray, Etc." or even adding trim tabs like these
http://www.boatownersworld.ca/smart_tabs/sx.htm.

When I took it out for the sea trial, it never did this, but also there was two of us and he weights about 280 (power lifter shape) and I am 220 (.......Shape). Could it just be a weight problem, too much in the stren and because it is aluminum, too light in the bow?

Any help would be appriciated.

Boat - 18" Hewescraft welded aluminum with a 34 gallon fuel tank, 115 yammy four stroke and 8 HT yammy 4 stroke. Also two batteries and downrigers, all in the stren.

Sorry for the long post.

 

[tyeegalore]

You probably had this covered when this started but was your 115 trimmed properly or the same as before when it happened?

 

[Gypseas]

Gonna ask the obvious... Have you tried trimming the motor down?
vic

 

[Gypseas]

TG beat me to it

quote:Originally posted by tyeegalore

You probably had this covered when this started but was your 115 trimmed properly or the same as before when it happened?

 

[salmon9]

Seems to me if it wasn't porpoising at lower speeds then you may be able to stop it by raising the trim on the engine. Factors affecting trim are: weight distribution, engine trim, speed, prop pitch/diameter, and external devices (like trim tabs). I am a fan of trim tabs since it's a more effective way to control boat trim without messing around with the other variables which often doesn't work (trimming the engine to manage porpoising may make require the engine to be less efficient - e.g. cavitate). Check out Lenco or Bennett trim tabs.

 

[Sitkaspruce]

Yup

Been on the ocean and running boats for most of my life, so I know the trim stuff and all that, just never had to deal with really bad porpoising. Makes for a really fun ride

 

[Sitkaspruce]

quote:Originally posted by salmon9

Seems to me if it wasn't porpoising at lower speeds then you may be able to stop it by raising the trim on the engine. Factors affecting trim are: weight distribution, engine trim, speed, prop pitch/diameter, and external devices (like trim tabs). I am a fan of trim tabs since it's a more effective way to control boat trim without messing around with the other variables which often doesn't work (trimming the engine to manage porpoising may make require the engine to be less efficient - e.g. cavitate). Check out Lenco or Bennett trim tabs.

Salmon9

It got a lot worse (almost uncontrolable) the more I raised the trim. The water was like Pi$$ on a plate, and it would do it at lower speeds as well, just not that bad. At 3300 rpm it was contolable and after a while you just got used to it, kind of....

Are those a big install trim tabs? The ones I linked to are really popular in the states and Alaska, but not sure about up here.

 

[scott craven]

Try a pair of Dole Fins before you go to the expense of the trim tabs. They are cheap, easy to install, and will help.

 

[Last Chance]

If it wasn't porposing before, is there a chance that your trailer put a hook in the hull? One of the few pitfalls of tin boats.

 

[Sitkaspruce]

quote:Originally posted by LastChance

If it wasn't porposing before, is there a chance that your trailer put a hook in the hull? One of the few pitfalls of tin boats.

Not sure what you mean by "Hook". If you mean dent, I will check, although not sure how it could happen as it is on 2x6 bunks. I will check it out though.

Thanks

SS

 

[Last Chance]

By hook, I mean a gradual dent. Maybe if the thing was coming off the trailer it see-sawed on the ends of the bunks or something. If this has never happened before, something quite drastic has changed in your boat. Could be as simple as the motor not trimming down all the way, does it look like it swings down right against the hull when it's on the trailer?

 

[Sitkaspruce]

Some more info to ponder

I started out of the hole with the boat in the lowest trim (closest to the boat) and got up on step, I then started to increase the trim, at around 2800, the boat starts to porpoise, backed off on the trim, boat settles down and plows water, increase rpm's and boat starts to porpoise again, no adjustment on the trim, yet. Try to adjust trim and the boat becomes a carnival ride. The best I could stand and still keep a decent speed (26 mph land) was around 3700 rpm, but soon backed down to 3300 as my back was getting a good work out. One thing I did notice was that there was three tabs still left on the trim tab indicator on the yammy guage. Not sure if they are supposed to go all the way down to none or stay at three. It will another thing to check out today.

I have had the boat out a couple of times and did not have this problem. I will check the motor to ensure that it trims all the way down, although I do not see how it will make a difference as I will then be plowing a whole lot of water. I will look into the extra fin as well. Any ideas on where and the best ones.

Again thanks for all the help so far...

Cheers

SS

 

[Last Chance]

On my rig, I had to find the sweet spot between trimmed down and up to avoid both porpising and plowing. Doesn't sound like anything wrong except maybe you are not running as much wieght in terms of fuel as you used to?

 

[Sitkaspruce]

LastChance

That is the only difference from other trips. I topped up the fuel tank. It was about 1/4-1/3 empty. I will try it again this weekend, weather permitting.

Thanks for all the help so far

SS

 

[redliner]

Sitka I have the same boat just wondered if the fixed trim tabs on transom have taken a hit somehow? . My yamaha controls show the same three bars when trimmed all the way down. Don't know if this helps but good luck

 

[daddystoy]

I think you might have answered your question in your first post. When you went out in the boat for the sea trial you had more weight in it. Porpoising, I believe is caused by the motor wanting to lift the boat out of the water like an airplane (so to speak) but once the boat gets to a certain height, gravity takes over and down it comes. This cycle continues until some variable is changed, i.e speed, weight or trim. By slowing the boat down it probably won't have the power to lift it out of the water as high. By adding or shifting weight to the bow this should probably keep the bow down. By trimming the boat so the bow is pushed down into the water should eliminate porpoising. I know how well trim tabs work. That would be my suggestion IMHO. For what it's worth

 

[tubilah]

We have the same problem on our boat. It started when we added the fins on the motor. We have no trim on our out board. We were thinking of getting trim tabs. On line we saw automatic bennet trim tabs They are a lot cheaper. Do these work as good as they say they do?