Porpoising. What should I do?

E

Eco Geeko

New Member
Hi all. I'm a long-time lurker, first time poster. I'm actuallys a life-long flyfisher, but recently I've been having a lot of fun getting into the saltwater and gear world. However, I've got a lot to learn about larger power boats.

I bought a used 16.5 Alumicraft with a 90hp Tohatsu last year. And I've had issues with it porpoising when on plane. The problem is so bad that I always have to back off on the throttle when on plane.

Here's an image and a couple videos of the problem. You can see the camerawoman has trouble maintaining position even when lying down at the back of the boat.

IZGqyk7.jpg

From stationary: https://imgur.com/wFZEkUY
Running: https://imgur.com/E17WaTb


I also think this boat should be faster getting out of the hole. The first video shows this. Not as big a deal as porpoising, but I suspect their both caused by the same problem.

I think the motor position looks decent when its out of the water (correct me if I'm wrong):

vJ1RzqB.jpg

hMLdMIJ.jpg


I'm not eager to pay someone to lift the motor on my hunch that it's mounted wrong, so I thought I'd consult the experts. What do you think?
 
Be sure to check the imgur links for video. I tried to edit the post so they're more clear, but that's getting blocked for being "spam-like"
 
The running pics/vid make it look like that motor is mounted way too low. You can do this yourself with a floor jack and some careful planning.

Have you got pics of the whole boat? It's hard to really know what's going on here. Porpoising can be due to poor weight distribution or also hull shape problems. You can try moving weight around in the boat and see what affect that has.

Are you reaching the proper full-throttle RPM of the engine with that prop?
 
motor position looks wrong. also likely need doel fins if you cant move it. does it ventilate (not cavitate) at all ?
 
It's hard to see the trim position, can you trim the leg closer to the boat?
 
My 18' maxum wasn't really even a functioning boat without the Dolphin.
 
If when you start porpoising you find that trimming the engine in does not help, you problem may be that the engine will not trim under far enough. You need to measure your transom angle http://www.seamount-mattson.com/trandiag.htm It should be in the general area of 12-14 degrees. If it is not you might not be able to trim your motor under enough.
 
I agree with the fellas that say you need to lift your motor after watching your video but looks can be deceiving.
Start with measuring your motor height to make sure it is mounted correctly.
Trim your motor so that the cavitation plate is parallel with the keel.
Take a straight edge and set it on the cavitation plate and up to the transom.
The cavitation plate should be 1/2 inch above the keel.

Google "16.5 aluminium boat motor rigging instructions" if you need more instructions.
 
Thanks all!

I can get a pic of the whole thing on the trailer. I don't have any pics of it in the water.

There's not much weight in the bow, which does push the nose way up under heavy acceleration. That's partly why it takes so long to get on plane for sure, but once there I have to back off on the throttle quite a bit to limit the porpoising issue. The most weight I've had in the bow has probably been 2 crab traps and 4 prawn traps and rope. That's not a lot of weight, but porpoising was still an issue. I suppose I could put a bunch of weight in one pf tre

I've certainly considered a hydrofoil (which I think is what ya'll mean by dolphin). I'm concerned that since the anti-ventilation plate always seems to be underwater that adding one might even exaggerate the porpoising issue, but I'm not very knowledgeable on the subject. Also, if the source of the problem is a bad motor mount, I'd like to address that first.

I haven't noticed any ventilation.

Here's a picture of the motor trimmed so the anti-ventilation plate is even with the keel, with a straight board sitting on the plate. (The trim gauge registers this as about 1/2 the total available trim. Its not that much, but its fairly significant). Thoughts?
NUU65He.jpg
 
That
Eco Geeko said:
Thanks all!

I can get a pic of the whole thing on the trailer. I don't have any pics of it in the water.

There's not much weight in the bow, which does push the nose way up under heavy acceleration. That's partly why it takes so long to get on plane for sure, but once there I have to back off on the throttle quite a bit to limit the porpoising issue. The most weight I've had in the bow has probably been 2 crab traps and 4 prawn traps and rope. That's not a lot of weight, but porpoising was still an issue. I suppose I could put a bunch of weight in one pf tre

I've certainly considered a hydrofoil (which I think is what ya'll mean by dolphin). I'm concerned that since the anti-ventilation plate always seems to be underwater that adding one might even exaggerate the porpoising issue, but I'm not very knowledgeable on the subject. Also, if the source of the problem is a bad motor mount, I'd like to address that first.

I haven't noticed any ventilation.

Here's a picture of the motor trimmed so the anti-ventilation plate is even with the keel, with a straight board sitting on the plate. (The trim gauge registers this as about 1/2 the total available trim. Its not that much, but its fairly significant). Thoughts?
NUU65He.jpg
[/QUOTE
That looks OK to me, others might disagree. As stated before if you cant fix it by trimming the motor a hydrofoil/ Doelfin might be your next step. You may already know this; trimming the motor down forces the bow down which should stop the porpoising and trimming it up lifts the bow. So you can trim the bow down for your hole shot and then trim it up until just before you porpoise for best fuel economy
Click to expand...
 
4 or 5 cannon balls in the bow locker helped for me. Adding fin also reduced porpoising, but more importantly brought the bow down much earlier on the hole shot. Seeing where you are going is nice (I'm sitting at the stern on a tiller). Am I the only one who thinks 90hp is too much for that boat?
 
I should have mentioned that I've played with the trim quite a bit with little effect on the problem. I've been really surprised how little the trim has done on this boat.

It's rated for up to 90hp. It's at the top end, but it should be good for it.
 
The motor height look good to me also. Motor weight may be an issue but we have to work with what we got. Bring weight forward may help. Dolfin may help but my experience with them is limited. I have changed a prop on mine that claims to offer stern lift. It worked for me but each boat is different. I used a "Turning Point Hustler 4 blade", it's aluminum so the price was reasonable. You would also need to buy a hub kit to match your motor. It fixed the porpoising issue that I had. Wedges may help but I have zero experience with that either. Here is a link to the prop I used, you can use the "find dealer" button to locate a dealer near you.

https://turningpointpropellers.com/
 
Eco Geeko said:
Thanks all!

I can get a pic of the whole thing on the trailer. I don't have any pics of it in the water.

There's not much weight in the bow, which does push the nose way up under heavy acceleration. That's partly why it takes so long to get on plane for sure, but once there I have to back off on the throttle quite a bit to limit the porpoising issue. The most weight I've had in the bow has probably been 2 crab traps and 4 prawn traps and rope. That's not a lot of weight, but porpoising was still an issue. I suppose I could put a bunch of weight in one pf tre

I've certainly considered a hydrofoil (which I think is what ya'll mean by dolphin). I'm concerned that since the anti-ventilation plate always seems to be underwater that adding one might even exaggerate the porpoising issue, but I'm not very knowledgeable on the subject. Also, if the source of the problem is a bad motor mount, I'd like to address that first.

I haven't noticed any ventilation.

Here's a picture of the motor trimmed so the anti-ventilation plate is even with the keel, with a straight board sitting on the plate. (The trim gauge registers this as about 1/2 the total available trim. Its not that much, but its fairly significant). Thoughts?
NUU65He.jpg
Click to expand...

Can you provide a pic with the engine fully trimmed down?

The hull looks dented in that pic. Is there some bottom damage? Hull performance can be very sensitive to what shapes happen at the transom: that's why trim tabs are so effective.
 
Motor looks too low judging by the stick picture.

It also sounds like you're trying to rectify multiple issues here that may or may not be related:
1. porpoising when running
2. slow hole shot

Start experimenting with weight distribution in the boat to address the porpoising. Get a passenger or two to sit at the bow when running and see if that has any effect.

Holeshot is going to be mostly related to engine power (is it running properly??) and propeller. The engine height (too low) will not hinder the initial holeshot. You also need to verify that your prop is in the realm of reasonable by checking the wide-open-throttle (WOT) RPM. If this is your motor:
https://www.tohatsu.com/marine/na/outboards/90_4st.html
It says that WOT RPM should be between 5300-6300RPM...generally you want to be in the upper half of the range, so that means that with a normal load you want to be 5800+RPM at WOT.

What I would do, in this order, is this:
1. experiment with weight distribution and see if weight forward helps the porpoising. If it does, see about moving battery & fuel further forward in the boat. You might even need a bit of ballast up in the anchor locker (a cannonball or two)
2. check engine WOT RPM. If you're not hitting 5800+ you need to change props
3. raise engine at least one mounting hole
4. re-evaluate performance
5. consider adding a cavitation plate. I like these or similar: https://www.davisinstruments.com/product/whale-tail-xl/
 
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