Porpoising while trimmed down

[Danny C]

I have a 17' checkmate enticer with a 115 Merc outboard on the back. I took it out for the first time two weeks ago and it would porpoise really bad at speed. I am trimmed all the way down and it has a hydrofoil which are the two things I keep hearing to do. I think my weight distribution was probably a bit off so I will try to correct that this weekend. I've also read to check my motor height. What should it be at? Last solution would be trim tabs which I kind of want to avoid just because I have limited space on the transome and it looks like they would get in the way of my transome straps. Should I go ahead and just get smart tabs? Any other suggestions? It seems odd that with a hydrofoil and trimmed all the way down I am still jumping.

Thanks

 

[Texasmark]

Re: Porpoising while trimmed down

Not familiar with your boat. Why don't you send us some pics of what you have. Side view of the hull, side view of the transom with engine in normal running position, rear view of the prop/lower unit area including the anti vent plate, interior shot showing stuff you have aboard and a frontal shot showing the bottom of the boat as the water sees it.

A pic is worth a thousand words as the saying goes. We'll be happy to help you and the pic can make the difference between success and failure. There are instructions on this site as to how you upload pics.....Don S. did the dissertation.

HTH,

Mark

 

[roscoe]

Re: Porpoising while trimmed down

First, check engine height.
Anti ventilation plate on motor, should be about an inch above the bottom of the hull.


Click thumbnail:




Also, I would try running without the hydrofoil.
You really shouldn't need it on that rig.

The engine must be mounted high enough so that the anti ventilation plate, and the foil, are out of the water when running on plane. If the lead edge of the foil is hitting the water, you are going to have problems.

 

[Danny C]

Re: Porpoising while trimmed down

Thank You guys for the replies. I will try to get my hands on a camera and post some pics. I have been toying with the idea of taking the hydrofoil off. I don't want to lose hole shot for skiing though. I will check the engine height and prop tomorrow morning. I could not see the attachment 106270. says invalid attachment.

 

[Triton II]

Re: Porpoising while trimmed down

I could not see the attachment 106270. says invalid attachment.

Roscoe, if the attachment referred to above is your thumbnail of the OB mounting explanation, I can see it loud and clear.

TII

 

[smokeonthewater]

Re: Porpoising while trimmed down

ditch the foil x1000.... it is almost definitely causing your problem..... Also if your prop is in bad shape that can also cause problems.... As for holeshot, that will be 90% prop selection. I doubt you will have any problem at all as long as you have an appropriate prop.

 

[Danny C]

Re: Porpoising while trimmed down

Hey Guys, I'm back with pictures. After my last post, I took the hydrofoil off and took it out with 4 people on board. Boy, it did not feel right at all without the hydrofoil (maybe I'm just used to the foil) seemed to be shaking a bit more and definately riding lower in the back, so much so that when I throttled up just a little and the nose stuck up in the air I stopped and my wake came over my transom. (my weight was admittedly back heavy due to the 2 male riders in back and tiny female in bow. I immediately went back to the dock. Luckily I thought to throw the foil in my car and slapped it back on. Instant improvement but now I am back to where I was. With the foil and good weight distribution I can manage but still porpoises. I find If I throttle back right after getting on plane I can usually control it for awhile but anytime I try to go fast I pogo across the lake.

So, as for my next steps, I checked the cavitation plate height and it is about 1-2 inches above the bottom of transom as you can see in the pictures (pic 6 and 8). The whale tale does stick down further but I dont really want to take that off after last time. What is the brackets with the different hole setpoints for (second pic in album). I am guessing it is to set your maximum amount you can trim in/down? Mine was in the second to last hole so I figure I can move it to the last hole or take it out all together and get a little bit more down trim. what is the hole for in fourth pic? The cresent hole is where the rod rests but why is there a hole there? Is there ever a reason to manually set your trim if you have hydrolic trim or is that just for if you do not have hydrolic trim? Last question is while the motor is trimmed down I can lift it manually. It isn't really easy, but its not that hard either. Should I be able to do this? I am thinking if I can do that then maybe on the water even when trimmed down the motor is being allowed to bounce up and down causing me to have no control over trim. I would think that the hydrolics should do a better job of keeping my motor where I set it.

I know that was long but just trying to be detailed.

 

[auwareagle01]

Re: Porpoising while trimmed down

??? don't see any pics?

 

[oops!]

Re: Porpoising while trimmed down

picasa page not found

 

[Danny C]

Re: Porpoising while trimmed down

I appoligize, bear with me I am new to Picasa.

TRY THIS LINK
I also added some more pics of under hull, prop, and transome angles. On second look, the tips of my hydrofoil definately dip even with if not below my hull. Should I raise the motor or once again try taking it off? Looked at my prop and could not see any markings on size/pitch. I now it is SS though.

 

[Texasmark]

Re: Porpoising while trimmed down

Hey Guys, I'm back with pictures. After my last post, I took the hydrofoil off and took it out with 4 people on board. Boy, it did not feel right at all without the hydrofoil (maybe I'm just used to the foil) seemed to be shaking a bit more and definately riding lower in the back, so much so that when I throttled up just a little and the nose stuck up in the air I stopped and my wake came over my transom. (my weight was admittedly back heavy due to the 2 male riders in back and tiny female in bow. I immediately went back to the dock. Luckily I thought to throw the foil in my car and slapped it back on. Instant improvement but now I am back to where I was. With the foil and good weight distribution I can manage but still porpoises. I find If I throttle back right after getting on plane I can usually control it for awhile but anytime I try to go fast I pogo across the lake.

So, as for my next steps, I checked the cavitation plate height and it is about 1-2 inches above the bottom of transom as you can see in the pictures (pic 6 and 8). The whale tale does stick down further but I dont really want to take that off after last time. What is the brackets with the different hole setpoints for (second pic in album). I am guessing it is to set your maximum amount you can trim in/down? Mine was in the second to last hole so I figure I can move it to the last hole or take it out all together and get a little bit more down trim. what is the hole for in fourth pic? The cresent hole is where the rod rests but why is there a hole there? Is there ever a reason to manually set your trim if you have hydrolic trim or is that just for if you do not have hydrolic trim? Last question is while the motor is trimmed down I can lift it manually. It isn't really easy, but its not that hard either. Should I be able to do this? I am thinking if I can do that then maybe on the water even when trimmed down the motor is being allowed to bounce up and down causing me to have no control over trim. I would think that the hydrolics should do a better job of keeping my motor where I set it.

I know that was long but just trying to be detailed.

Not being a rebel rouser, but you need someone to teach you how to drive your boat. You may be amazed at the problems it solves. BTDT. Sold my '89 Ranger to my son-in-law.....disasterous performance. I took him out and showed him the ropes and he has had it ever since....pushing 10 years; loves it.

Mark

 

[Danny C]

Re: Porpoising while trimmed down

Hey TexasMark, I do not disagree with you, in fact that was going to be one of my suggestions "maybe I am not driving it correctly"...I have only taken this boat (my first) out a few times. My only previous experience has been renting boats once or twice a year. Mostly deep hull ski boats and sometimes pontoons, but never a shallow hull like I have. That being said, after getting on plane with trim all the way down, what could I be doing wrong, operational wise, to cause this porpoising? It happens all the time, calm and rough water and pretty much thoughout the throttle range unless I am going pretty slow.

 

[Bounce19712]

Re: Porpoising while trimmed down

trim all the way down for out of the hole shots....only.

trim up after getting out of the hole.

you'll notice when trimming up after planing the rpms should come up a bit, the boat is not plowing bow down as much


I've never driven with trim full down unless pulling a skier out of the water...then I trim up to improve performance...

but you've prolly already tried running in various trim....

 

[Danny C]

Re: Porpoising while trimmed down

Thanks for the advice Bounce. To be honest I have been driving it at full down pretty much the whole time because of the porpoising and I have always heard trim down if porpoising. I did try trimming up once at speed while porpoising and ended up cavitating. Although I believe it is because I trimmed up too much as I don't have a guage and couldn't gear the trim while running and couldnpt tell much just be looking at motor while trimming. Next time I will get up to speed and try to nudge the trim up in small incriments and wait a few seconds inbetween increments to see if performance improves...I guess it may just go back to don't know how to properly drive. But again, I know trimming all the way down gives poor performance but could it be creating the porpoising by digging into the water causing the bow to bounce?

 

[Maclin]

Re: Porpoising while trimmed down

We need to see some pics from the side looking at your engine trimmed all the way in framed to where we can see the cav plate in relation to the bottom of the boat, and to get an idea of the angle the engine is at when trimmed in all the way.

The links go to picasa but then picasa says it cannot find the page. If the pics are on your PC then just use the Image icon to upload with the post.

 

[Danny C]

Re: Porpoising while trimmed down

Sorry, I thought the link worked...ok, apparently picasa sucks at posting links. If anyone wants to see the whole album of the boat and has a gmail account let me know and I can send you a link. I did not want to just post pics on the thread b/c I know some sites don't like that, but I am at a crossroads here. If posting pics on the thread is frowned upon here either let me know or feel free to remove them, but please dont lock the thread as I would like to figure out my problem...

Here are some pics:

 

[Danny C]

Re: Porpoising while trimmed down

more...they are all a little out of order, but here they are...

 

[ezmobee]

Re: Porpoising while trimmed down

Motor looks to be mounted a little high to me. (I'm far from an expert in this area though)

Typical starting point is with the cav plate even with to about an inch above the keel. You're way above that. I'd try dropping it down to about an inch above and remove the hydrofoil and see how it behaves.

 

[ziggy]

Re: Porpoising while trimmed down

Motor looks to be mounted a little high to me. (I'm far from an expert in this area though)

that's where i'm at too. including the part about me not being an expert either.
this is my i/o...

in the bottom pic, the cavitation plate is the lower fin. ya can just see the start of it. w/o measuring it. i'd say it's about an inch above the keel.

also agree in removing the whale tail. i know it's helping you now. but i believe it's masking some under lying issue. like the o/b being mounted to high.

 

[Maclin]

Re: Porpoising while trimmed down

Motor is too high and that is the reason the prop catches air when trying to trim up. Like Ziggy and others are saying, the whaletail is masking the real problem.