Boat planing, speed issue

[Pete41985]

Hi all - so I am new to owning a boat. I just got a 1975 Lund 16' jon boat with a 20 HP Mercury from my wife for my birthday - awesome gift! I took it out one time a couple months ago and cruised around with my buddy and dog. It cruised at a pretty good clip, a hat-backwards kind of speed.

Then, my wife wanted some storage in it. So the father-in-law (experienced boat owner) and I used some treated 2x4 and 3/4" plywood and built a deck in the front. Added a couple pedestal seats to the boat as well. Now, it's about 50 extra pounds all together, and that's being generous. I've taken it out twice with the new set up and the speed just isn't there. The disparity from what it was to now doesn't make sense for an additional 40-50 pounds, at least to me and a lot of other guys I've talked to.

It's at full throttle, motor is running great, sounding great, starting right up and never quit s. But it doesn't want to plane and just pushes water at the front, slogging along. It's embarrassing, really. A few times when first starting and opening up the throttle it planes and seems okay, then just goes back to pushing & slogging.

I changed the gas, added some additives and made everything is running correctly. Could it really be the weight making that huge of a difference? I had my 220 pound buddy and my 85 pound dog with all their gear in the front of the boat before and it zipped right along. But my 160 pound dad & my dog sitting in the middle and it still doesn't get up to speed or plane.

Please help!

 

[Bondo]

A few times when first starting and opening up the throttle it planes and seems okay, then just goes back to pushing & slogging.

Ayuh,..... Welcome Aboard,.... That sounds like water in yer gas,.....

 

[gm280]

:welcome: aboard pete41985. Great to have you join us...

Okay the first thing I see is the treated 2 x 4's. Treated wood and aluminum boats don't get alone real well. The solutions they use to treat the lumber will seriously cause aluminum corrosion and quickly! With that stated, I can see that build up changing the hull performance because of the weight. Most folks use 1 by material and thinner plywood. So you probably don't think you put that much weight up front, but you actually did. And you can prove that to your self if you remove it and see. If you are wanting to install things like that, you have to understand how quickly weight adds up... With that added addition, now your engine has to try and lift that extra weight just to plane off. And the fact that the extra weight is in the front of the hull makes the engine work that much hard. So I'm not surprised it doesn't plane anymore... Look at it like this. You have a pole, you add a five pound weight on the pole by your hand. Not much more effort to keep it horizontal. But slide that five pounds out to the end of the pole and see how much hard it is now to keep the pole level. You just did that same experiment with the engine... JMHO!

 

[Pete41985]

Thanks guys. Glad to be here!

It's a fiberglass boat and I did switch out the gas before yesterday, and put in the fresh non-oxy with some additive. I am beginning to agree that it's the weight, as much as I don't want to after that big project! It's just hard to believe that the extra weight could be that drastic! Like I said, my buddy and dog were up front prior to the project and we cruised around at a pretty good clip.

Is there anything else that could be done to salvage the project? I tried bringing up the trim one hole and it helped just a bit, should I bring it up another? I'd hate to take it all apart, and my wife would never want to go fishing!! Haha.

Thanks for your advice!

 

[Grub54891]

Well, make it removable, so when the wife wants to go, put it in, fishing has nothing to do with speed.

 

[smokeonthewater]

Absolutely work with the trim... Keep moving up till performance stops improving and then move back down one notch...

Ditch the 3/4" ply and make a new deck from 3/8"...

You built that front deck strong enough to hold about 2000 lbs

If you don't want to rebuild the framework at least remove the lower supports... They aren't needed...

 

[Scott Danforth]

your working on a boat, not a house. the structure you built is way overbuilt. I would have used 3/8" ply and 1 x 3' or ripped 3/4" plywood. you added about 100# looking at the pictures, not to mention the treated wood vs aluminum corrosion issue.

I suggest pulling the structure, testing, then adding weight until you hit the performance limit. then when you know the real weight you can live with, build the structure to that weight. you may have to use nida core and build a fiberglass honeycomb structure.

 

[bonz_d]

As I recall that nice old Lund has a maximum capacity of about 1000lbs with engine and people. By adding that deck, pedestal, seats You just lost 100+ lbs. of that capacity before adding engine and people.

My opinion also is that you over built that deck. Small boats are very weight sensitive and it doesn't take much to throw off the balance. As with the case of small aluminum boats were people move the fuel tank as far forward as possible to keep the bow down.

2 other options would be to try a smaller pitched prop or move to a larger engine.

 

[Pete41985]

Thanks for the tips - sounds like I have a few things I can try, but the moral of the story is this deck has been way overbuilt. Thanks again!!

I will report back with what the trim levels do for it, as well as deck removal.

 

[gddavid]

what is the engine capacity for the boat?, maybe you could sell yours and buy a used 30hp or so.

 

[gm280]

Just remember, a 16' and wide boat is probably already at the planning limit before you improved the front section. So see what the capacity plate states. If it is close to your engine size, you will have to remove some weight. If it stated it can handle a 40HP or 50HP or more engine, then leave the new addition and look for a bigger engine...

 

[Pete41985]

The engine capacity is 25HP, and I have the 20 on it now. Here's another pic of what it looked like prior to the update.

 

[smokeonthewater]

I think you'll find that adjusting trim gets you going again and of course shedding a few pounds won't hurt.