Getting rid of porpoise/getting boat dialed in.

[496 Cubic Inches]

I have a 20' Aluminum boat with an offshore bracket and a Yamaha F150. At speeds less than WOT I can't trim my motor out all the way or it starts to porpoise. I can easliy rid the boat of porpoise if I either power thru the bounce or trim it way back down.<br /><br />At WOT I can trim all the way out and the boat handles great. At less than WOT I have to have it trimmed out less than halfway or I get the bounce again.<br /><br />Am I correct in that I should be able to trim it up fairly far at less than WOT? Or is it standard to have to be running at WOT to utilize all the trim?<br /><br />With the standard Yamaha aluminum prop (14x19) I was able to utilize all the trim and the boat handled great. I needed to gain about 8-900 rpms so the dealer gave me a Yamaha Saltwater series SS prop (15 1/4x17). After I switched props I gained the rpms and performance needed,but also got all the bounce.<br /><br />Does prop style contribute to porpoise? <br /><br />I'd like to get my boat all dialed in,and the dealer will continue to let me test props until I'm satisfied. What should I look into next?

 

[WillyBWright]

Re: Getting rid of porpoise/getting boat dialed in.

Stainless props produce bow lift. It's in the rake of the blades. Apparently your boat doesn't need it. You can tame-down the porpoising with trim tabs. You have to adjust trim most anytime you change the throttle, that's why they put the trim switch in the throttle lever. It's perfectly normal.

 

[Dhadley]

Re: Getting rid of porpoise/getting boat dialed in.

Never, ever go down in pitch to gain rpm until the set up is maxed out. I cant imagine having to go to a 17 on a 150 on a 20' aluminum boat. I realize the 4 stroke is lazy but you should easily be able to swing a 19. Especially with 2:1 gears. Unless the aluminum boat has a house on it.

 

[Texasmark]

Re: Getting rid of porpoise/getting boat dialed in.

My experience with boat setup is exactly as you say. When your performance is so good that you porpose and can blow out of it, you are running a well oiled machine.<br /><br />You answered your own question. If you want to run slow, trim her in till she's smooth. If you want to blow out; trim her all the way out and put the pedal to the metal. Grin. When you slow back down just touch the trim till she's steady at that speed.<br /><br />We could attempt to go thru the process that causes the porposing, but it is a balance thing. When the engine is trimmed out it tries to push the bow up and there is a lot of load on the prop (caused by transom drag.....the weight of the boat is on the hull at the transom but the water pressure isn't pushing against it as much as it needs to) which causes prop slip.<br /><br />This slip slows the boat slightly, causes the bow to drop, which brings the transom up thus reducing the drag and the boat speeds up slightly, which puts the transom back down and on and on. Or something like that.<br /><br />Sometimes, I like to deliberately trim out when dead in the water and like the pogo ride porposing out of the hole. With the rig I have now it only takes about 3 or 4 cycles and she is steady as a rock, blowing and going.<br /><br />Mark

 

[496 Cubic Inches]

Re: Getting rid of porpoise/getting boat dialed in.

Dhadley,<br /><br />With the 19P prop I could only get to 5K rpms,the F150s operating range is 5-6K. I was at the very low end of the range. You say you don't like to drop pitch to gain rpms,what else can I do to get to the upper end of the operating range?

 

[walleyehed]

Re: Getting rid of porpoise/getting boat dialed in.

I'm not Dhadley (as you can see)but why don't we look at raising the engine on the transom, or pod?? Get the drag factor lower, pick-up some speed and the RPM will come too.<br />There "could" be a better prop for that application as well, but I like the large diameter...that allows a higher mounting heigth..<br />OK, let's see what DH has in store for ya....

 

[Dhadley]

Re: Getting rid of porpoise/getting boat dialed in.

The answer is in your first post. "The dealer gave me.....". Thats the quickest, safest, easiest, least work thing he could do. The best thing he could do for his customer in the long run is to work on the set up. Or give advise to you rather than just say "here ya go".<br /><br />I'd have opted to go with a ss 19 and raise the motor like Walleyehed mentioned. Anytime you lose drag and increase efficiency you'll gain from holeshot to wot. Lose drag to gain rpm to gain speed and efficiency.<br /><br />Now, in the end you may indeed have ended up with a 17 but I just can't believe that 4 stroke is that weak. Maybe it is. Or perhaps the boat is a lot heavier than I suspect.<br /><br />I know folks who have aluminum boats in this range with 150's that use a lot more prop than that. And they have a 1.86:1 gear ratio, not 2:1.<br /><br />Walleyehed, do you know of any boats like that?<br /><br />But, back to your handling question -- thats normal in some cases. Over trimming at less than wot should cause the boat to porpoise. You dont have enough prop speed to carry the hull. Not uncommon in performance hulls. Thats what makes me think you could be doing better. The boat is asking for less trim or more prop speed. Simply pushing a button or a lever cures the problem. By advancing the throttle to settle it down, some folks consider that "driving thru it".

 

[Texasmark]

Re: Getting rid of porpoise/getting boat dialed in.

Yeah, D, and "driving thru it" is fun too! Right?<br /><br />Matter of fact, I have had so much fun playing with this newly acquired boat (er ah toy) that I probably ought to leave my fishing rods at home. They don't get used much. Oh well. Grin<br /><br />Mark

 

[Dhadley]

Re: Getting rid of porpoise/getting boat dialed in.

Mark, I figure that "testing" is as good an excuse....errr....reason to go boating as fishing is. Or skiing or cruising or....

 

[496 Cubic Inches]

Re: Getting rid of porpoise/getting boat dialed in.

Originally posted by Dhadley:<br /> <br /><br /> Or perhaps the boat is a lot heavier than I suspect.<br /><br /><br />

I haven't had the boat across the scales yet,but I'm estimating its close to 3500lbs. <br /><br />The boat is a North River Seahawk with an offshore bracket. Its not the typical Smokercraft,Lund,Alumacraft type Aluminum boat. Its made more for offshore use and is fully welded and uses heavy gauge aluminum construction. I wouldn't consider it a performance type boat. <br /><br />I'll try to get some pics of what I have.

 

[496 Cubic Inches]

Re: Getting rid of porpoise/getting boat dialed in.

Here is a picture of what I have.

 

[gatorred]

Re: Getting rid of porpoise/getting boat dialed in.

that like a fishing boat the trim has to be set right or it will porpoise to high will do it mine bass boat dose coming off plain, off shore boat's are racing boat type i thinking of, your's is a nice boat!

 

[Dhadley]

Re: Getting rid of porpoise/getting boat dialed in.

Very nice looking rig!<br /><br />Interesting bracket. Does the bottom of the bracket go clear down to (and match) the hull?

 

[496 Cubic Inches]

Re: Getting rid of porpoise/getting boat dialed in.

Actually that boat is a 21'er. On the 21' the bottom of the offshore bracket is an extension of the hull. They designed the bracket to add bouancy to the 21' as alot of guys are running 225s on them and they get a bit stern heavy with that much motor.<br /><br />I have a 20' model thats identical except for the bracket,its more of a traditional O/S bracket. Its not an extension of the bottom.

 

[496 Cubic Inches]

Re: Getting rid of porpoise/getting boat dialed in.

With the F150,I'm getting about 45-46 mph out of her now. I don't know how much more speed this style hull will take.

 

[Dhadley]

Re: Getting rid of porpoise/getting boat dialed in.

OK, so the bottom of your bracket goes forward to the transom and then the transom goes down to meet the hull thus creating a step. Right?

 

[496 Cubic Inches]

Re: Getting rid of porpoise/getting boat dialed in.

Originally posted by Dhadley:<br /> OK, so the bottom of your bracket goes forward to the transom and then the transom goes down to meet the hull thus creating a step. Right?

Correct

 

[gatorred]

Re: Getting rid of porpoise/getting boat dialed in.

is it even at the bottom of boat where the bracket is bolted on or is there a step there,my boat has live well pickup that make mine have a built in step area there.

 

[496 Cubic Inches]

Re: Getting rid of porpoise/getting boat dialed in.

Originally posted by gatorred:<br /> is it even at the bottom of boat where the bracket is bolted on or is there a step there,my boat has live well pickup that make mine have a built in step area there.

The bracket is not flush with the bottom of the boat. There is 5" between the bottom of the bracket and the bottom of the boat.

 

[496 Cubic Inches]

Re: Getting rid of porpoise/getting boat dialed in.

I'm working on a better pic of my O/S bracket