Help get my boat running true!

[Sabbath]

Just in need of some opinions from both camps.

I have a 14foot Fibreglass boat with a 115hp Chrysler Outboard on it. Im suffering terrible bow-rise on takeoff and unbearable porpoising at cruise.

The bow-rise i can live with to an extent but it's the porpoising that i really want to get rid off, which product would be better,and why?

Smart Tabs

or

Hydrofoils

Attached is a picture of the boat

and a Video of the ride quality

Thanks guys.

 

[oops!]

Re: Help get my boat running true!

porpoising is caused by several problems.
trim tabs will help.....but the real; problem will remain.

lets investigate this before you shell out boocu dollars on a set of tabs for a 14 footer.

check the angle of the anti cav plate in relation the the bottom line of the hull.

or better yet, tale a pic of the motor full down and let us see !

one of the problems, i think you are way over powered for that boat

 

[Sabbath]

Re: Help get my boat running true!

Thanks for the quick reply oops!

Here is the best pic i've got at the moment (night here so would mean moving two cars, opening shed and moving the boat forward so it will have to wait for now) showing the cav plate in relation to th bottom of the hull. The hull is very progressive on the side so im not sure if that might be causing some issue aswell.

Here is one of the back of the boat so you might be able to visually swing it down in your head

My friends boat (the red one) is running a 105hp chrysler on a similarly sized boat, and is running foils and has no porpoising etc but sits very flat so im not sure if that's the hull doing that, or the foils. I realise it's a big motor, for a small boat and im never going to get a perfect result but understandably im after the best possible.

 

[etracer68]

Re: Help get my boat running true!

We would need a side pic showing the bottom of the boat and the anti vent plate on the motor in full down position. It should be level with the bottom of the boat. If you have power trim, maybe tipped slightly up on front of anti vent plate. This will plane quick, then trim the motor out, or up a bit.

 

[Sabbath]

Re: Help get my boat running true!

As asked for, here are the pictures:

This is from the underside of the plate. 2-3mm from the underside of the plate. When i measure from the top, it's exactly even

 

[NSBCraig]

Re: Help get my boat running true!

Your motor does look low, start at the anti ventation plate 1" above the bottom of the hull. But then that's probably why the aluminum contraption is back there, someone was tryin to get it right.

This will help all around performance, but not really porpoising. You should buy some smart tabs. Their only $100 so I don't know where boocu comes from. Do a search on here and you'll find countless threads of people telling you how well they work, and check the Smart Tabs website it explains it all.

I don't know why you would be overpowered (or way overpowered) either lots of boats that size are rated for a 115 at worst your 15 maybe 20 over, but you'll have to check the capacity plate to know that. That's not what's causing the porpoise problem either way.

Cool little boat.

 

[H20Rat]

Re: Help get my boat running true!

pretty much agree with the post above 100%. It looks a little low, but not horribly. Probably could go up a little bit. I'd also invest in some smart tabs. They are pretty much meant for boats exactly like that.

 

[etracer68]

Re: Help get my boat running true!

From what I see is, it doesn't look like its trimed all the way down. Cant tell from the pic. Put a long straight edge on the bottom of the anti vent plate, and see if its parrell with the bottom of the boat. If its (anti vent plate) tipped up in the back, its not down far enough.

 

[Sabbath]

Re: Help get my boat running true!

Thanks for the replies guys. Really appreciate it.

The motor was installed like that by the last owner. I dont really want to be moving it any if i can get away with it. Motor is 115 and transom/boat is rated to 130, so it's still within the limit.

Looks like i'll be purchasing some Smart Tabs then opposed to Hydrofoils then?



This is the bow rise on acceleration:

 

[Bondo]

Re: Help get my boat running true!

This is the bow rise on acceleration:

Ayuh,.... What bow rise,..??

This is bow rise,.....

 

[spoilsofwar]

Re: Help get my boat running true!

Ayuh,.... What bow rise,..??

Agree, I saw less bow rise then I'd even expect would be normal on such a small boat with a big outboard hanging off it.

 

[spoilsofwar]

Re: Help get my boat running true!

Sorry for back to back posts, but I paused the video and grabbed a screen shot at the point where it looked like the bow was at its highest point..



Seems reasonable to me.

 

[Scott Danforth]

Re: Help get my boat running true!

I agree that the bow rise is minimal, however in most cases, if your porpoising, you should trim down just a bit. looking at the picture in post#5, it looks like the motor needs to be trimmed in a bit.

 

[Willyclay]

Re: Help get my boat running true!

Your friend's boat, in addition to having a hydrofoil on the motor, also has a deep-v hull and a motor that is matched to the transom height. Your motor is a long-shaft model on a short-height transomed boat that has a flat, racey hull with mini-tunnels and non-trip chines. While Smart-Tabs probably will help, an adjustable jack-plate might be the ticket instead of that fabricated transom extender. Finally, your motor does not appear to have PT&T which may be the single most helpful method of controlling bowrise and porposing. Good luck!

EDIT: The link below will let you see diagrams of what Mercury says about mounting height.

http://continuouswave.com/whaler/reference/engineMountingHeight.html

 

[Sabbath]

Re: Help get my boat running true!

Ayuh,.... What bow rise,..??

This is bow rise,.....

Challenge Accepted!

Agree, I saw less bow rise then I'd even expect would be normal on such a small boat with a big outboard hanging off it.

The boat isnt currently running right, i've had problems with it cutting out when it goes into gear and i've only just got it to the stage where it will go without cutting out. So the holeshot isnt asmuch a holeshot it's more of a bog down and gentle climb. It used to take off better with a 19pp and i've gone down to a 17pp and with the issues i've experienced a slower holeshot. But i'll be sorting out that issue next. A definate winter issue to fix. Luckily we dont need to wrap boats here in Australia.

I agree that the bow rise is minimal, however in most cases, if your porpoising, you should trim down just a bit. looking at the picture in post#5, it looks like the motor needs to be trimmed in a bit.

Mmm, yeah the trim motor is getting a bit on the old side i think, it starts to labour at the bottom, and top of the travel

Your friend's boat, in addition to having a hydrofoil on the motor, also has a deep-v hull and a motor that is matched to the transom height. Your motor is a long-shaft model on a short-height transomed boat that has a flat, racey hull with mini-tunnels and non-trip chines. While Smart-Tabs probably will help, an adjustable jack-plate might be the ticket instead of that fabricated transom extender. Finally, your motor does not appear to have PT&T which may be the single most helpful method of controlling bowrise and porposing. Good luck!

EDIT: The link below will let you see diagrams of what Mercury says about mounting height.

http://continuouswave.com/whaler/reference/engineMountingHeight.html

Thanks for the info. Will check that link out now.



Thanks to all for the replies

 

[etracer68]

Re: Help get my boat running true!

Have you tried the straight edge? It doesnt look like theres any trim pin in the hole I talked about. Your motor problems, may be carbs, or timming not advancing properly.

 

[Frank Acampora]

Re: Help get my boat running true!



Hmmm! I see you Aussies like Chryslers! Don't make me come down there! LOL

Here are a couple of photos of a 14 foot flat bottom with a 125. It would be similar to yours without the two air entrainment tunnels. NOW to the good stuff.

This boat used to porpoise badly with a 105 doing only 38 top speed. Now, with a 90 or a 125 topping out at 55 and 60 respectively, it is rock solid, without trim tabs. On the hole shot, with the trim tucked all the way in, it will give one or two bounces and be planing at 45. As I feed in up (or out) trim, the RPM and speed rise until max up trim is reached.

NOW: You have a little boat with a big engine and she is tail heavy--even though the Chrysler is the lightest over 100 HP 4 cylinder around (276 pounds without power trim). With all that weight in the back, she is going to want to bounce. You need to get some weight forward. Look closely at my photos. See how stable she is at speed? Notice that the battery is located directly behind the driver's seat and the 15 gallon fuel tank sits crosswise there too.

You MAY be getting flexing of the bottom. I was, and I installed another reinforcing cross brace about 2 feet in front of the transom

You also need to go to the prop section and ask about a STERN LIFTING prop. Regular props and cupped props are bow lifting and will aggravate an existing tendency to porpoise. A stern lifting prop will tend to raise the transom and level the boat. More hull will have air under it and speed should increase. Notice in my first two photos how little of the hull is wetted and the level attitude of the hull even with full up trim. My next experiment will be with side deflecting rails on the bottom to try and raise the hull higher and increase speed to 65.

With your 115, I think a good stainless 21 pitch prop should be close to correct with just one person in the boat. It should allow the engine to reach its rated RPM --4500-5500-- and give good performance.

However, none of your photos adequately shows: You ABSOLUTELY MUST have power trim and tilt. Setting the engine to a single position will not do justice to that boat and will undoubtably decrease performance. Hole shot and top speed will both be compromises.

 

[Frank Acampora]

Re: Help get my boat running true!

And by the way: That Chrysler will be happiest with the anti-ventilation plate (cavitation plate) set to one inch above the bottom with the plate directly horizontal to the bottom.

Your friend's 105 has a 2 piece lower unit which sits 1 inch deeper in the water than your one piece lower unit (the midleg is one inch longer) AND his lower unit is about 3 MPH slower (for a given horsepower). That right there will account for a lot of his stability. Also, if he ever could reach 60 MPH which is highly doubtful, that lower unit will "blow out" meaning the nose shock wave will be larger than the prop diameter and it will lose all thrust.

 

[Sabbath]

Re: Help get my boat running true!

View attachment 142211View attachment 142212View attachment 142213View attachment 142214

Hmmm! I see you Aussies like Chryslers! Don't make me come down there! LOL

Here are a couple of photos of a 14 foot flat bottom with a 125. It would be similar to yours without the two air entrainment tunnels. NOW to the good stuff.

This boat used to porpoise badly with a 105 doing only 38 top speed. Now, with a 90 or a 125 topping out at 55 and 60 respectively, it is rock solid, without trim tabs. On the hole shot, with the trim tucked all the way in, it will give one or two bounces and be planing at 45. As I feed in up (or out) trim, the RPM and speed rise until max up trim is reached.

NOW: You have a little boat with a big engine and she is tail heavy--even though the Chrysler is the lightest over 100 HP 4 cylinder around (276 pounds without power trim). With all that weight in the back, she is going to want to bounce. You need to get some weight forward. Look closely at my photos. See how stable she is at speed? Notice that the battery is located directly behind the driver's seat and the 15 gallon fuel tank sits crosswise there too.

You MAY be getting flexing of the bottom. I was, and I installed another reinforcing cross brace about 2 feet in front of the transom

You also need to go to the prop section and ask about a STERN LIFTING prop. Regular props and cupped props are bow lifting and will aggravate an existing tendency to porpoise. A stern lifting prop will tend to raise the transom and level the boat. More hull will have air under it and speed should increase. Notice in my first two photos how little of the hull is wetted and the level attitude of the hull even with full up trim. My next experiment will be with side deflecting rails on the bottom to try and raise the hull higher and increase speed to 65.

With your 115, I think a good stainless 21 pitch prop should be close to correct with just one person in the boat. It should allow the engine to reach its rated RPM --4500-5500-- and give good performance.

However, none of your photos adequately shows: You ABSOLUTELY MUST have power trim and tilt. Setting the engine to a single position will not do justice to that boat and will undoubtably decrease performance. Hole shot and top speed will both be compromises.

Thanks Frank. Always love hearing your knowledge, especially Chrysler related. That other boat belongs to a member on here "Spool" who had the thread later last year about rebuilding his 105hp.

The motor has the hydraulic tilt control on it, but it seems to get a little slow at the end of the travel and almost strain against itself to pull into a full upwards/downward position. 21pp will be out of the budget unfortunately for a little while but i may consider it for the future. Next time i go out im definately going to experiment with setting the weight out in the boat better. I currently have a 25 litre removable tank that i stick in the bow and the battery sits behind me. BUT. While i've been having these problems i've been carrying a second battery and a second 25litre fuel tank which sit in the stern adding another ~30kg/60lbs to the stern.

Atleast this gives me a nice list to do over winter

 

[Sabbath]

Re: Help get my boat running true!

Double post, but...just went out to test to see if i could get it to tilt down any further to make the cav. plate any flatter. But i found that the way i have it at the moment, is as flat as i can get it. So not sure what's causing it. But if i havent been able to get it trimmed out flat when im running then possibly another cause of my symptoms?