Clunk noise with flat bottom wooden boat

[DuckHunterJon]

Hi all, I have an interesting issue with a wooden boat I built two years ago. The boat is 12' 6" long with 4' beam. I'll attach a picture or two. It has a perfectly flat bottom, ecept a skag that runs from the front of the boat (about 1" tall) to roughly 75% of the way back (about 3" tall at the back). I currently have a 1964 Mercury 39 (3.9 HP) on the back.

When WOT, the boat won't plane off unless I move up to the middle seat. When it is plowing through the water at WOT, I hear these intermittent clunks on the bottom of the boat, feels like it is right under my feet ahead of the back seat. It sounds like hitting something, yet I know I haven't. I'm thinking it's some kind of cavitation due to the flat, smooth bottom, but just curious if anyone else has ever seen something similar. I really don't pay much attention to it anymore, just curious.

As for the motor/planing issue, I've passed up on several 5 and 6 hp motors simply because I love the classic looks of the old Merc. Yesterday I came across a Merc 59 (5.9 HP) on CList that I'm going to check out tomorrow. It looks like the same vintage, and if in good shape, I'm interested to see the performance difference.

Thanks for any insight comments anyone might have.

 

[Bob_VT]

Re: Clunk noise with flat bottom wooden boat

The bow is swallowing air and you hear the pockets of air passing below you.

Have you considered a tiller extension for your motor allowing you more freedom towards the middle of the boat???

 

[steelespike]

Re: Clunk noise with flat bottom wooden boat

If your not too heavy and have some weight to stow forward you may be able to plane with the 5.9 If your heavier maybe not.
Years ago we had a steel boat that resembled yours with a 5 hp Bucaneer.
With 180lbs in the rear nothing, with a tiller extension it popped on plane almost in a boat length.It approached the max rated 5hp speed of 12 mph.

 

[Lion hunter]

Re: Clunk noise with flat bottom wooden boat

^^^^^+1 on the air pockets. Every flat bottom boat I ever had did that. Might try throwing a 5 gallon jug of water in the bow to see if that solves your problem. Or get a fatter retreiver for the duck boat.

 

[DuckHunterJon]

Re: Clunk noise with flat bottom wooden boat

Thanks SteeleSpike. I'm about 160 lbs, and it will almost plane off. But add in a hunting buddy (180 lbs), my lab (90 lbs), 2 dozen decoys, 2 guns, 4 boxes shells, etc - and it's pretty slow. My goal in building it wasn't a speed racer - but I'd like it to be at least as fast as paddling speed.

I did pick up the 6 HP Merc yesterday. Typical CList story. Talked to him on the phone, he said it hasn't run in 2 years. Show up and the story changed to "Uncle Joe died 8 years ago and he didn't run it in his last couple of years". He was asking $250, offered him $150 and brought it home. Looks in pretty nice shape. Dropped it in a barrel of water, conected up the gas line, primed it. On the second pull, it coughed. On the third pull it was running like it just ran yesterday. No water coming out the exhaust, so I shut her down and will pick up an impeller today. Anxious to see how the boat goes with it.

Fun fact - the Merc 60 and Merc 39 turn out to both be Phantom Paint models, both 1966's, and with in 1000 of each other on the serial number!

 

[DuckHunterJon]

Re: Clunk noise with flat bottom wooden boat

Final post of the results. So the weather was too nice to be mowing/planting garden/staining the deck/or any of the other mountain of chores to be done last night. Got home, hooked up the boat, and headed for a small creek I like to visit.

With the "new" 44 year old 6 HP coming off a fresh water pump service (ended up changing impeller, all seals, and guide tube), I was anxious to see the performance difference.

Just for data, the boat weighs 160 lbs with out the motor, I add another 160, and I had my copilot with me adding another 90. I took both motors with me for testing. I put on the 6 hp, and took off. Well, crawled off is probably more like it. I was expecting a bit more going from 3.9 to 6, but it just wasn't that much different. Checking with GPS (yeah, I know, I need to get a life!) - the difference betwen the motors was 4.6 mph WOT vs 5.8 mph WOT. The boat still didn't plane off. Both motors run excellent, the prop on the 6.0 could be a little better, but I don't think it's too bad.

I'm still considering it an upgrade though. The motor starts and idles so much better than the single cyl. Don't get me wrong, the 3.9 runs great, but it is a single - every time it coughs on initial start up, it will stall as it doesn't have another power stroke to bring it around again. It won't idle well until it warms up. The twin just purrs. I'm also more confident in the water pump as it has a real tell tale vs the 3.9 just spitting a bit of water out the exhaust hole. Lastly, the 3.9 has no kill switch, so you kill it by idling all the way down. I like being able to drop the idle to minimum with out fussing with it on the bigger motor, then hit the kill button.

Overall, I'm very happy with the "new motor", but will be keeping my eye out for a Merc 7.5 down the line. Sorry for the long story, hope someone finds it a useful comparison.