Thompson with nose-diving problems

[jody sanders]

18' Thompson, built in Peshtigo in 1962 (serial number 12616).
After 20 month restoration, we had new 90 hp Evinrude E-Tec installed last weekend. At approx half throttle, bow will dive, and I must pull back on throttle, immediately. Boat comes up on plane very easily, but the bow is pushed down at approx 18 knots.
Motor is properly installed, with correct (20" version) and trim mechanism will not overcome this tendency to nose dive.

This boat was stored on trailor for eight years, and had some hogged areas. We worked the stem out and secured it with inboard longitudinal brace, and modest keel, approx 4" at deepest point. But there are still some areas aft that seem to be slightly concave. I am at a loss to know what is going on here, but would like to get an original drawing, so I could compare present situation to factory specs.
Any ideas, or advice would be greatly appreciated. I am only 2 hrs from Port Townsend, WA, so that could be an option, for expert help.

[rbgarr]

Have you asked the Thompson owner's group?
http://www.thompsondockside.com/

[Thorne]

You may want to contact the Thompson association, although Andreas may respond to this thread. Please post pics of the hull mods, keel, motor in place, etc.

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[wizbang 13]

ANY concave in the after 8' (bottom, of course) in that boat will cause exactly what you are talking about. If you have already done a bunch of work to make the bottom strong, don't sweat it. You could fair the bottom.Sounds like, turn the boat upside down and get the back/bottom sorta level. grind the aft 8 feet, and fair with epoxe. Run an 8 ' (aluminum) straightedge around to find gaps. Use the same straightedge to apply and "float" the goo. 407 up to 1/4" gaps, switch to 410 for 1/8 or less. Then if you REALLY want the boat to go, put the engine on a bracket!! But seriously, don't spend money on propeller or whatnot till the bottom is faired.

[jody sanders]

Thank you Wizbang 13. I wonder if I could do the work you suggest without turning the boat upside down? Removing engine, fuel tanks, etc might be a bit of a problem for me, but working upside down would be a challenge for my 65 year old body, too. If I could get it level, etc, could we apply the 407/410 upside down?

[AndreasJordahlRhude]

Yup, sounds like there is a hog in your boat. Now how did Porky the Pig get into your boat like that? Poor storage is the most likely culprit.

Even the slighest amount of hog can cause crazy handling at planing speed. I've seen it with only 1/4 to 3/8 inch hog.

The 4 inch deep keel is NOT factory original. So...someone along the way has modified the boat.

There are NO original drawings available.

What is the hull ID stamped into the wood of the transom, inside the boat? This is different than the serial number on the rectangular metal builder's plate. The hull ID will give the model year, in secret code!

The serial number 12616 falls within about the 1958 model year. However, as stated above, the hull ID will give the year.

Factory picture at Peshtigo circa 1963-64 showing about an 18 footer:
http://i232.photobucket.com/albums/ee189/ThompsonBoat/ThompsonJig08.jpg

Andreas
thompsonboat@msn.com

[jody sanders]

Thank you, Andreas. Here is the only other set of numbers I've been able to find, and they are on plate @ transom WAZ 7 424 A 1962
457701
Yes, the keel piece was added by me, along with stainless transom bracing. We plan to use this boat in salt water for fishing, cruising, and crabbing, and have outfitted it with new custom 20 gal (ea) fuel tanks, etc. It is definitely no longer a stock boat, but remains beautiful.
I would send you pics, but am not sure how to do it on this site.
You are exactly right on improper storage....she sat on a trailor for 8 years, with bunks falling 4" short of transom. Bad deal, as I've come to find out.
We are thinking that working from underneath, we can sand, and apply epoxy as leveling filler. (I have a tech call in to Systems Three). Turning the boat upside down is not an easy option for me, so I hope the goo can be applied upside down.
Is there a way I can send you pics? I'm not that savvy concerning the site instructions and warnings.

[wizbang 13]

The reason to go upside down is to save work, not to add it. Sanding properly is almost impossible upside down, and I'm a so called expert! This is not just a patch . At least get her off the trailer and roll her on to her side, but overhead will take all the fun out of it .

[seo]

If it turns out to be a concave section right aft, would you want to repair the hull so it's straight, or just fair it with putty?
Often the cause of the concave section is that the boat sits on a trailer, supported on the hull by the trailer bunks/rollers, but the full weight of the engine and transom cantilevered out there with no support.
My GUESS is that it can be repaired by supporting the weight of the stern, and then pushing out the hull to its correct shape. You might break some ribs, but oh, well. An additional floor timber or two back aft might help, but always remember that the hull on dry land is a very different engineering propositiion from when it's in the water.
If you want to try fairing out the concave just to see if that's the problem, you could use just about any putty for a temporary fairing. My choice would be a polyester putty called "Hull and Deck" which is used by fiberglass boat shops a lot. I'd probably put a few small wood screws into the bottom to give the putty something to hang onto, and to give you a thickness guide to help you get the surface flat and fair.
Then take her out and try it. I would not be surprised if the hull tries to settle back into its proper shape after a while in the water, and this may cause the fairing patch to come off. Which might not be a bad thing.
In any event, no matter which fix you employ, don't let the motor's weight cantilever off the back of the trailer.

[seo]

My logic for suggesting Hull and Deck over epoxy or vinylester is that it's cheap, kicks off quickly, and is pretty easy to use.

[seo]

Here's a couple of sources for hull and deck:

http://www.mertons.com/Epoxy/polyester_Resins/putty.html

http://store.hamiltonmarine.com/browse.cfm/hull-deck-putty-5-gal-pail(use-mekp-hardener)-117943/4,16702.html

It's not hard to find. I wouldn't use automotive "bondo" because that stuff is not waterproof, or very strong. Well, maybe for a very temporary fairing it would be okay...

[John B]

Can't you adjust the outboard trim?

[AndreasJordahlRhude]

The "WAZ....." ID is a state assigned code. It was not put on the boat by the manufacturer. It hs nothing to do with the year of the boat. As I stated earlier, serial number 12616 falls within the 1958 year range, however, it can be verified from the hull ID, which differs from the serial number. Don't ask me why Thompson at Peshtigo used a serail number and a different hull ID - they just did.

I can also tell the difference between a 1958 and a 1962 Thompson of Peshtigo boat thru photos. The hull shape and many details are very different between those two years.

You can email photos directly to me if you desire: thompsonboat@msn.com

To post photos on this site, you first have to post them to someplace such as photobucket.com and then cut and paste 'em into your message.

Andreas

[Typhoon]

How much does the new engine weigh compared to the old one? Any major weight shifted/added during the build?

Regards, Andrew.

[ccx2]

Can't you adjust the outboard trim?

That would be my first adjustment also, not hard to do and may make the difference you desire without a whole lot of unnecessary hull work.:):)

[AndreasJordahlRhude]

In private emails with the owner, I am fairly certain that the boat is a 1958 Thomspon Bros. Boat Mfg. Co. (Peshtigto, WI) Off-Shore with centerline length of 18'-1" and beam of 82 inches.

The boat (no motor or fuel tanks of junk in her) weighs 960 pounds (originally) and she was rated for up to 100 HP motor and 1,950 pounds weight capacity.

Do NOT get rid of the hull ID on the boat in the restoration process. It makes a proper ID pretty difficult without it.

The boat has a hog in the bottom. So this is most likely the culprit for the wild handling issues.

Andreas

[Thad]

No mention has been made of the pitch of the engine. Can't this have this effect? If the prop is in effect pushing up on the stern this will push the nose down as you power up. If the prop is pushing down it will tend to raise the bow. Changing the attitude of the engine is one of the factors in tuning the engine to the boat.

[Ian McColgin]

I'm not a propeller head but I remember these boats from my youth and they all seemed to have a little hollow hog - made them plane easily with the power available then.

But you do have a hog in the boat. It's the mill. This hull is suitable for speeds up to maybe 20 knots but beyond that, it's not really designed to fly. If you modify the bottom to get the bow back up a little, you'll set yourself up for an inelegant back flip.

Get a smaller motor and enjoy life.

G'luck

[AndreasJordahlRhude]

This boat is rated for up to 100 HP. The Thompson Bros. Boat Mfg. Co. factory did not build a hog into the hull.

On my 1957 Thompson of Peshtigo 16'-7" Sea Lancer, it had a 110 HP Mercury (1967) on the transom when I got it. Probably reached speed of 38 MPH. and handled it no problem. I switched to twin 1957 Johnson 35 HP motors because I wanted the same year motors as the boat. My top speed now is probably in the high twenties.

Andreas

[GrassSlipper]

Hog seems to be the general consensus,
but plowing can be attributed to an unbalanced load also.
Too much weight forward can cause the same problem.

[htom]

There are many possible causes and the boat in question might have more than one. The hogging is the first thing I'd deal with, though, because all of the other modifications attempting to compensate will cause other problems -- and won't fix the nose dive.

[pcford]

Andreas has the right answer.

I think the correct term for this condition is a "hooked" bottom rather than hog. It is typically caused by sitting on a trailer which is too short to support the weight of the engine. You should always check for this on Thompsons, Cruiser Incs, Lymans and similar boats.

I have dealt with this by building a frame inside the boat that pushes the bottom out. Don't like the idea of just filling the hollow with goo.

Regarding the supposed motor adjustment cure: The boat will always have a hook in the bottom no matter how the engine is adjusted.

[ChaseKenyon]

one common temp (good for 20 years or more) fix for Thompsons and GWs with trailer hogg is to lay in some nice 1/4 thick 2 in angle iron 8 to 10 feet long each side of the inner keel rib. Through bolt through the bottom, caulk and seal it all with boatlife or equivalent. On the 19 footers here in New England this was standard practice as the almost all hogged. If the hogging is severe like 4 inches at four feet from the transom add some floors.

On one 19 footer that had sent 10 years outside on a roller trailer I used four floors of 2 inch thick white oak cut to fit under the standard deck panels. They forced the keel down resotring the sectional shape of the bottom to original design shape.

That is a quick and simple fix. It also can be done with the boat still on the trailer and is much cheaper than trying to microballoon the hog out.

On FG Glaspar G3s the factory put a hook in for the last three feet to put the boats up on a plane soooner with the max 70 hp two bladed motors of the time. With modern HP and props most restored G3s have had the last 8 ft of the bottom filled and leveled. If not at speeds with a 115 or more outboarad they develop to much lift and go squirelly.

THink about the Thompson fix, and contact me if uyou need more info on it. I have done 4 or 5 this way and worked on at least ten that have had it done before me.

[floatingkiwi]

Someone might have said this but shouldn't ya try the easiest stuff first. Like lift the engines trim a bit,i.e. out and away from the transom, and even try one of those metal plates that extend the surface area of it the anti cavitation fins. The horizontal ones just above the prop. I had a boat that frightening would do this aswell. It was an inflatable without a rigid bottom. I did not own that boat for long after that, nor have I had another since.
' Try the easy stuff first mate.

[pcford]

Someone might have said this but shouldn't ya try the easiest stuff first. Like lift the engines trim a bit,i.e. out and away from the transom, and even try one of those metal plates that extend the surface area of it the anti cavitation fins. The horizontal ones just above the prop. I had a boat that frightening would do this aswell. It was an inflatable without a rigid bottom. I did not own that boat for long after that, nor have I had another since.
' Try the easy stuff first mate.

Why don't people bother to read the thread before they post?

[Bill Huson]

FIX THE BOTTOM! Get rid of the hook. Engine trim is used to adjust the attitude of the boat to suit the conditions and desires of the boat pilot. Trim is not used to correct an uncorrectable flaw in the bottom shape.

Keeping the last few feet of our race boats bottoms perfectly flat was extremely important. We would slather a thick coat epoxy on, or epoxy with filler if needed, and then "stink stone" it. Stink stone is a large (2' x 2' or so) few inch thick block of blown glass insulation obtained from a heating supply place. Hydrogen sulfide was used to blow the glass into a cellular-like structure, so when you pushed the block back and forth, the glass bubbles would break and release the main component of stink bombs. Hey, it was a smelly job, but the stink stone would grind the epoxy bottom dead freaking flat.

A power boat with any sort of hook or rocker in the aft portion of the bottom is an accident waiting to happen. Fix it, please. As my Pappy use to say: You can't walk from a boat wreck.

[wizbang 13]

Go to "scream and fly" (forum) to see the importance of bottom fairing. Fix the hook first.

[floatingkiwi]

my mistake.

[MiddleAgesMan]

My Simmons has the intended "hook" aft but it's only about 3/8", maybe 1/2". I certainly hope the sea trials don't reveal anything like this.