19 foot Offshore Power Dory Build #15: Perfomance Update.

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Blog entry by English posted 09-18-2015 09:33 PM 4636 reads 0 times favorited 7 comments Add to Favorites Watch
« Part 14: Finished trim and fit and launched. Part 15 of 19 foot Offshore Power Dory Build series Part 16: Crack in Fiberglass on Transom »

I wanted to update my blog to let folks know that I had performance issues with my flat bottom designed boat. I believe I caused the issue by adding so much weight forward in the cabin and pilot house. The boat had a real control issue. It would not steer well at low speeds. The boat would just seem to slide sideways instead of turning. In reverse there was no control at all. The front of the boat would fish tail back and forth banging the dock on the way out of the ramp.

I contacted the hull designer, Jeff Spira of Spira International Inc., and discussed the issue with him. He recommended that I add a strake down the center on the hull bottom. He suggested a 1×4 – 6 feet long mounted on it’s edge, starting at the transom running forward 6 feet. After discussions with him on how to mount the strake (I call it a Keel) we decided on a 2×4 – 6 feet long on it’s edge, and due to possible cavitation with the motor it was moved forward starting 3 feet from the transom.

I discussed how to mount the piece with Jeff and with several other folks. I was first looking at a piece of white oak for the job, planning on epoxying it to the bottom and fiber-glassing it for durability. Jeff told me he would not epoxy or Fiberglass the piece. He said that I would be changing it from time to time due to wear.

It was recommended by the Dealer at my local Yamaha store that I use a structural plastic 2×4 for this job. I found one available locally, I purchased it and made my “keel”. The front is rounded and the aft end is tapered back to 1”.

I then installed it to the bottom of the boat using Stainless steel #14×5” structural screws that I ordered online. I put a screw every 8” down the length of the Keel, sealed it to the bottom with 3M 5200 adhesive sealant.

I took the boat out today and it performed great. Backing out of the ramp was perfectly straight. Turning at slow speed works great now and turning at higher speed is much improved. The difference is like night and day. I actually felt like I was in control of the boat today.

Here is a video of the performance testing. The boat is riding a little high and I plan to work on that. If I get it fixed I will update the blog.

I did go back and look into the reason for the high bow on plane, and decided that I needed trim tabs to bring the bow down, I decided to give the Nauticus Smart tabs a try. If they work I would not need any more controls.

They worked better than I had expected. They not only brought the bow down but they shortened hole shot and let you handle wakes much better.

Thanks for looking!!

-- John, Suffolk Virgina

7 comments so far

View htl's profile


5464 posts in 2274 days

#1 posted 09-18-2015 11:43 PM

Cool project!!!
Your boat just needed pointed in the right direction. LOL

-- An Index Of My Model making Blogs

View grizzman's profile


7836 posts in 4418 days

#2 posted 09-19-2015 01:12 AM

well im glad you found the solution to the problem, always something to learn, and now you have a great running boat….enjoy the days of boating ahead…john, i want to ask if you can talk on the Physics of this and how it now changes the dynamics of the water flow.

-- GRIZZMAN ...[''''']

View shipwright's profile


8727 posts in 3913 days

#3 posted 09-19-2015 01:58 AM

Great news John. You deserve a great boat and now it seems like that’s what you have.
Well done, now get out there and enjoy!

-- Paul M ..............the early bird may get the worm but it’s the second mouse that gets the cheese!

View English's profile


691 posts in 2592 days

#4 posted 09-19-2015 02:14 AM

Bob, here is the best explanation I could find to explain how the keel works. The following is a quote from another source.

” A large part of the resistance of a keel is created by the vortices, similar to miniature whirlpools that form when the water flows across the bottom of the keel from the high-pressure (leeward) side to the low-pressure (windward) side. It requires energy to form those vortices and that energy is then not available to propel the boat forward. Obviously, the shorter the keel or fin tip, the smaller and weaker those vortices will be, and that translates to reduced resistance. “

You see these vortices forming behind a paddle as you pull it through the water and the water rushes to fill in behind the paddle.

-- John, Suffolk Virgina

View lightweightladylefty's profile


3654 posts in 4827 days

#5 posted 09-21-2015 03:20 AM


As I started to read your post, my heart just sank. (At least it wasn’t your boat that sank.) But what a nice solution and we’re so happy that you got it worked out successfully. Thanks for sharing your experience and insight with us.


-- “Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety.” Benjamin Franklin -- Jesus is the ONLY reason for ANY season.

View farmerdude's profile


682 posts in 3154 days

#6 posted 02-16-2016 10:45 PM

Well English, I finally put two and two together. I know you have left comments on my canoe blog and added a few tips. I got thinking about that today and looked at your home page and found your blog. I’ve spent quite a while this afternoon reading the whole blog from the start. It is an interesting read. Now I know where your knowledge came from. You have paid your dues for sure. I guess the canoe doesn’t seem like that big of a build compared to your boat. You put an amazing amount of work into that project and have a fantastic looking boat to show for it. When I did my first canoe, I too learned the hard way not to mix up the resin and hardener jugs. It wasn’t anywhere near as big an area as you had, but it taught me to pay attention. I really enjoyed watching the boat take shape. Shipwright was right when he said watching a build was almost as exciting as building one yourself. You did a fantastic job of taking pictures and writing enough information to keep me interested. I don’t care for a blog that only has a few pictures and no information to explain what I’m seeing. It may be foolish, but I am curious about one thing. There is a piece that I believe you called the splash guard for the motor, when you filled the screw holes, the material you used is a lot darker than the filler you used on the rest of the boat. Did you use a different wood flour, or some other thickener? Not a big deal, it just caught my eye. Anyway, you made one heck of a beautiful boat. I enjoyed your blog, and thank you for an afternoon of enjoyable reading.

-- Jeff in central Me.

View English's profile


691 posts in 2592 days

#7 posted 02-17-2016 12:14 AM


I have thoroughly enjoyed reading your blog. I hope to build a cedar strip canoe this summer for my grandson. So I have been watching your build very closely. As for the Dark Filler on the splash well, you have a good eye. I used Douglas fir flour on the hull and topsides but used pecan flour on the splash well. I ran out of DF and had a quart of pecan I had bought to get an idea of the texture of the “wood flour” needed for filler.

Thanks for the nice comments. and I will keep watching your build with great interest.

-- John, Suffolk Virgina

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