The bow bob's up and down onplane, Why?

[ntoracn@yahoo.com]

Just purchased a 1992 17' Blue Water boat with the 4.3LX/ Mercruiser drive. When I plane the boat and I try to trim the bow up at all, the boat begins to build up this bobbing action. If I trim the bow down, the problem goes away.

What is this called and what causes it?

Thank You in advance.

 

[fredreesey]

Re: The bow bob's up and down onplane, Why?

Porpoising is what it is and it happens when you trim the boat excessively. Do just as you did, ring the bow back to where it does not do this and you'll be fine. Can also happen if you forget to put your plug in and the read of the hull gets ful of water. Don't ask me how I know that.

 

[oops!]

Re: The bow bob's up and down onplane, Why?

fred thats funny....welcome to i boats...


and yes...its called porposing.......

you also might try to distribute the weight in the boat better...?

is the cooler in the back?...live well in the back full of water?

a 400 pounder sitting in the jump seats?

the weight must be distrubituted a little better.

 

[oops!]

Re: The bow bob's up and down onplane, Why?

this is a quote from jb...one of the moderators here.....

it is about how to use the tilt and trim properly. it may also help you.


JB says......

Once you get on plane trim the engine out until it is just short of porpoising the boat or ventilating the prop. Your speed and rpm will increase until you reach this point, then either the prop will start sucking air and over-revving the engine or the bow will start to hop up and down.

When you have found the ideal trim you can back off the throttle to your preferred cruising speed.....

hope that helps you
cheers
oops

 

[Jack Daniels]

Re: The bow bob's up and down onplane, Why?

So what would be the best solution if you dont have power tilt and trim?

 

[ntoracn@yahoo.com]

Re: The bow bob's up and down onplane, Why?

Thanks guys. No live well on this boat, no passangers behind me and no water inside the hull.

It just seemed that I shouldn't have to keep the hull level with the water to prevent this porposing effect. Is this what you guys have also found with your boats? My last boat (14' Gregor Aluminum fishing boat) would also do a little of this if I trimmed the motor to get the bow way up.

Anyways, it was our first trip out on this boat to do a shake down run, I have plans to use it alot this summer (I live in So. Calif) and will be able to fine tune the boat with a bit more time with it.

Thanks again

 

[wca_tim]

Re: The bow bob's up and down onplane, Why?

....Can also happen if you forget to put your plug in and the read of the hull gets ful of water. Don't ask me how I know that.

if you happen to forget the plug.... and don't notice until the boat is looking pretty low in the river... Its better to take the cell phone off your belt before you grab the spare plug and go over the side to put a halt to the flood... unless of course you really wanted a new phone anyhow...

I had forgotten that particular incident... thanks for the laugh!

 

[pduquette]

Re: The bow bob's up and down onplane, Why?

Hello J.D. (I'll Sorta hijack the thread) You 'll have to go through the process manualy. Tilt your OB forward as you would while trailering . Move the locking pin (horizontal bar the motor clamps too) out to it's last hole. See how the boat handles . Probably will porpise. Keep tilting the OB and moving pin in and trying the rig out untill the porpising disappears ,and then the boat starts to plow the bow under. when that happens set the lock pin back out to the previous setting. that will be your sweet spot. Taking data while you test will help greatly -speed (by GPS is perfered), RPM , and seconds to get to plane are most helpfull. Then you can begin adjusting motor height to fine tune even further....hope this helps

 

[180shabah]

Re: The bow bob's up and down onplane, Why?

Lets not forget this is a 16 year old boat with an unknown history. Hull deformation due to improper storage/trailering can cause excessive porposing.

Grab a straightedge and place it on the bottom of the hull. It should be flat for the first few feet in front of the transom. Any convex(rocker) or concave(hook) areas need to be inspected and repaired.

Not saying this is definately your problem, but checking it is free.

 

[Jdeagro]

Re: The bow bob's up and down onplane, Why?

Even new boats porpoise, but hull damage or configurations can make it worse.

Porpoising is usually caused by boat balance. Too much weight in the stern area. This make the boat run lite in the bow. When the motor is trimmed down the stern is lifted by the prop and results in more bow pressure. This often works to minimize porpoising but it also tilts the prop an inefficient angle. Results are a loss of speed at any RPM level (not just top speed), and loss of fuel economy.

The best bet for overall improvement in ride, handling, performance and economy is to install trim tabs. Do some research!

 

[ntoracn@yahoo.com]

Re: The bow bob's up and down onplane, Why?

Thanks "180", The boat is flat on bottom without any damage or deformed areas.

Jdeagro, I think you have it (I hope). Once I get some time on this boat, I think I will figure it out.

Thanks for your help.