bad handling of 16 foot '61 Thompson

[Scott5]

Hi all,

I'm the relatively new owner of the boat listed in the subject line. We had her out once this last summer were I discovered some scary handling properties.

She's powered by a 1961 75HP. Even with a bad dog clutch and a questionable state of tune, this motor was capable of pushing this boat to point where she'll suddenly "dive" and cut a steep turn to one side of another. This forces me to gradually reduce speed and coast to a stop.

To keep her running at high speed (i.e. maybe 80% of throddle) I felt like I was balancing on a tightrope.

What's the scope? - simply a characteristic of the boat? Am I simply trying to push her faster than she can go? - If that's the case, I'd hate to see what she did with a fresh 75HP! And why is this craft rate for 90 HP?

Or is something else afoot? - I've read about a warping these boats can experience.

Thanks,
Scott

[nedL]

Sounds like you might just have to much power, but there are two things to check first. 1) How is the outboard adjusted. Can you tilt the engine any farther back (moving the lower unit away from the transom). This will bring the stem up & make her ride better. 2) Has the bottom hogged at all from sitting poorly on the trailer? What this means is that when you site along the bottom of the boat from the transom foreward does the bottom (along the keel)run in a straight line, or does it rise up some as you move away from the transom and then come back down farther forward (this is called hogging). Hogging of even an inch or so can cause her to ride nose down. Just a couple of thoughts.

[ 01-20-2004, 12:57 PM: Message edited by: nedL ]

[Scott5]

Thanks Ned.

I gave a guy at a fingerlakes marina a couple bucks to go with me for a ride. He did adjust the motor as far back as possible as you suggested. This helped a great deal and allowed the boat to come up on plane instead of just plowing water for awhile before getting out of the hole.

But it only helped slightly with the sideways pitching - not enough to make me comfortable. The marina gent indicated that this was a characteristic of "all these old boats".

I'll check for hogging - I think the boat has spend a good part of it's like on a trailer.

What's the worst that can happen when a boat of this size does "dig in". Can the boat actually flip itself or anthing that drastic?
I'm sure the boat's going less than 30 MPH, but it feels like 110 with eventhing going on!

Looks like it's time for a honda 50 HP! With the money I save on gas and oil, a 4 stroke will pay for itself in one summer!

[WWheeler]

...not familar with the Thompson, but equivalent runabouts of that vintage (Peterborough for example) were typically powered with a 20-30hp MAX., and it was common to have a 10hp. I spent most summers with a 5hp. Maybe you're just WAY overpowered.

however, I have seen a restored Peterborough with a new 50hp Honda, and that was a sweet looking unit. A wood boat is so quiet, with a 4-stroke its even quieter.

[ 01-20-2004, 04:17 PM: Message edited by: WWheeler ]

[Noah]

I haven't driven a Thompson, but I have had our 18 ft Penn Yan and our 16 Lyman out and going fast. I have never had this problem in either of these two boats, except when I was being towed in the Thomson.

The puller was going too fast and I had no way to cut the rope that quickly...

The boat started going side to side, and it was very scary.

I think that the bow was too low and we were generating a very large bow wave...IE not getting on plane.

They shouldn't do this, and you should be able to get going pretty quickly in that boat. I would think that you should figure out why the boat isn't getting on a full plane. Tweak the engine, pull out the 200ft of chain in the front, etc...

And check for Hog.

Noah

[MuddyFeet]

Interesting thread....I have a 16' Cruisers (very similar boat) that does the same thing. However, it's powered by a vintage 40Hp Johnson. I would hardly say it's overpowered -- I have to run wide open to stay up on plane with a skier. (Not that it's an issue for the old Johnson! They never die...)

But I never figured out how to avoid the behavior you're describing. It seems to occur when the boat falls off plane or the bow gets into a trough. It then digs in and swerves off as you describe.

You'll notice these lapstrake runabouts have narrow sterns and the bow has a deep v in the forward section. At rest, the lowest point of the hull is just ahead of the windscreen (water collects in the bow). Compare that to a modern plastic hull -- it will have a wider stern and the centerline is flat from bow to stern (water collects at the stern).

My theory: That low spot in the hull will pull the boat off plane. When the bow digs in, it has nothing to give it directional stability and will pull the boat to one side or the other.

If you look at a Chris-craft of the same era, you see the same hull shape. But I've never experienced this in a Chris-craft. My theory there: The higher powered inboard engines keep the boat on plane through the chop, not allowing the bow to dig in.

All theories...no facts. :) But it's interesting that someone else noticed the same behviour.

[bob goeckel]

from everything i've seen and read about this problem it's probably hog. even a small amount acts like a trim tab angled down too much pushing the transom up and the bow down and on a lapstrake boat like this it causes the lower forward keel to act like a rudder. it's a scary feeling too. also a bit of work to fix. if you got to the thompson(thompsondockside.com) website forum you'll find info on correcting it. the sideways pitching is accentuated by prop torque while driving through the water. if the torque pitches you to port then try putting as much weight as you can to starboard(gas tank, battery,aunt martha etc.) it was a problem with the older boats

[nedL]

It sounds like there isn't to much for me to reply about now (things have been prety well covered). Your "marine gent" was pretty correct when he said it is a common characteristic. I used to have a 1954 13.5' Old Town runabout (smaller & older version of yours) with a 1961 25hp Gale(OMC) outboard and I understand exactly what you are talking about, especially when crossing a wake at an angle. I don't know if the boat might actually roll over when it starts with that routine, but it sure make one uneasy. Try to keep all the weight aft to help reduce the tendancy.

[Scott5]

Thanks everyone for the excellent feedback.

I'll take a look for the hogging. All weight is already aft and if the hogging isn't something that can be reasonably fixed, perhaps I'll go for a lower HP and enjoy a more mild boating experience where the sideways dive doesn't scare the daylights out of me!

I'm almost looking for an excuse to get a smaller, quieter, power plant. I *really* feel guilty everytime I start that olf monster Johnson up and see the oil slick form behind me!

[bob goeckel]

yea but think of all the water yer calming down for the next boater.

[Frank Mehaffey]

I have a 16.5 Lyman, with a '00 40hp Merc. The problem you described, started happening to us after 3 years with the faster, more powerful motor. Our bow would pitch down, and pull to the left, and the motor would start to cavitate. What had me stumped was the fact that we did not have that happen the first few years it was on the boat. This would happen only with light chop, at full throttle. With the short shaft motor, and the back coming up, it seemed to pull the prop up a little too much.? I took a dremel and rounded off the leading edge of the prop, where we had some minor nicks, and that seemed to correct 75% of when it happened. I think the combination of the nicks bringing bubbles, and the prop getting too much air, pushed us over the edge. In flat water, or heavier chop, nothing.I may try a 4 blade prop next season, instead of the factory prop, if it crops up again.

[whb]

Frank

I regularly boat on a shallow river.

Every season I take a file and true the blades to each other and smooth out the leading edge.

The difference in the smoothness of the ride is pronounced.

Course the side effect is that I wear away the blade enough that I have to replace it every couple of years.

Howard

[Andreas Jordahl Rhude]

HOGGING is your problem almost for certain. As Bob G. said, even the lightest amount of hog will cause serious handling problems. Your boat is certainly NOT overpowered. I have a 110 HP Merc on my 1957 Thompson Sea Lancer with a centerline length of 16'-7" and she handles like a dream. Your 1961 16 foot Thompson (if built by Thompson Bros. Boat Mfg. Co. of Peshtigo, WI) is probably the Sea Coaster or Thomboy model and they are rated for up to a 75 HP motor. The centline length on these models is 15'-10"

What's the hull ID of the boat (stamped into the wood of the transom inside the boat) and serial number (stamped onto a small rectangular aluminum plate)? (different numbers if a Thompson Bros. Boat). What is the maximum HP and weight ratings on the redish OBC plate?

Have you checked out www.thompsndockside.com (http://www.thompsndockside.com) yet?

Mr. Andreas Jordahl Rhude
Founder - Thompson Antique & Classic Boat Rally

[Scott5]

Thanks for the info Mr. Rhude,

The boat's in storage so I don't have quick access to the serial numbers, but I'm pretty sure I remember seeing the rating at 90 HP.
I also got 2 johnson 75HP seahorse engines with the boat - one presumed to be origional - so it makes sense that the boat *should* be able to handle this power.

My boat was actually made, I believe, in courtland, NY.

I did try a search at Thompsondockside.com but wasn't able to search the forum very effectively. The term hogging got me a link to a restored boat picture and to a pair of archieves containing hundreds or thousands of messages - so no - I don't really have much info on hogging - or how to fix, yet.

[Andreas Jordahl Rhude]

If she was made at Cortland, NY the hull ID code is completely different than that of Thompson at Peshtigo, WI. The two operations split in Jan. 1959 becoming competitors. Cortland, NY became Thompson Boat Company of New York, Inc. while Peshigo, WI remained Thompson Bros. Boat Mfg. Co.

An article in a past issue of "The Thompson Dockside" newsletter explained one method to repait hogging. It is not published on-line, you'd have to purchase the paper copy from the publisher.