Conversion from trailer-sailer to low power motor boat

Welcome to the forum Doug.

The typical shapes used in sailing craft wouldn't benefit from a wedge, in regard to getting more speed from them. It's simple physics really and nothing short of reshaping the entire after end on the boat will help.

You also may want to start a new thread about this subject as there are some tricks that do work, besides wedges.

 

Hi Par, Thanks for that I will set up a new thread I have only just found this forum and it seems like a gold mine of info and that there are some other Mad boaties like me out there.

 

Most aren't willing to publicly admit to the madness, but yes, you're quite correct.

 

New to the forum but this subject caught my eye. I have owned all types of boats but have narrowed down my needs to a motor sailor that leans toward motoring more often, is efficient under power and can get into tight relatively shallow area's. I set off to price materials to build a boat to fit my parameters and realized how expensive that was so I started researching older boat hulls out on the market. So why not convert a sailboat, I tried to find small sailboats 25 to 30 feet that would lean toward more of a power boat stern (flatter and wider), a low rocker hull to keep the hull speed up, Trailer able because I like to travel around. Swing keels ruin interiors, full keels are hard to launch and trailer so I added shoal draft keel to my list. What I found is many of probably more unattractive sailboats as far as lines and sailing performance fit the into my list. I found a few choices but, Don't judge me! I ended up picking up a American Mariner, very roomy hull, wide flat stern, shoal draft keel you can actually put your batteries and water tanks in. Considering there reputation when they where made I am really surprised at the construction, most of the area's I would call questionable as far as construction and design I have cut out. You can pick these boats up for nothing! I have cut the deck off, and gutted most of the hull. My biggest hurdle was try and get the esthetics up a notch. I am adding raised bulwarks forward, redesigned the interior and cabin top, adding a swim step/motor mount with a bottom that will be a large trim plate. Finished design will be 29'6" +-, powered with a 30 to 40hp, be functional on and off the trailer, launch time including rig less than 1/2 hour. Boat is 50% complete at this time. There are tons of small sailboats in all sizes that are great candidates for power. Really love some of the designs I have seen above, is anyone else doing this out there, I would like to here your thoughts.

 

here is an interesting add, I am sure there are some candidates. http://inlandempire.craigslist.org/boa/4298415597.html

 

Hi Kevin

It has turned out to be a big job but I feel I have got over the worst. I have attached pictures of the transom before and after the modification.I haven't taken many pictures because I see her everyday. After posting this some time ago and listening to the advice from the masters I felt I needed to learn more.so I did more research and went through "Lindsay Lords" paper on planing hulls which opened my eyes. When modifying the hull I feel I have it on the edge now between semi displacement and planing.

I am down in New Zealand and have never seen an American Mariner.I searched on the net and found some pictures and you are right has a wide straight transom and not a lot of rocker but still looks to be Full displacement and also has a round chine. Have a very close look at the MacGregor 26 that's the hull shape you need for more speed than standard hull speed of about 7 knots and you will also need a lot more power than a 30 or 40hp. The other problem is the weight of the boat and the drag on that keel. The MacGregor uses a water ballast system that can drain out when planing.

The boat in my Avatar I modified a few things raised the cabintop moved the boom up modified the transom to take a motor centrally and rudder system etc. I ended up with a very slow motor boat and not a very good sailboat.

What I am trying to say is keep it as a sailboat and tick around with a nice 9.9hp 4 stroke or modify hull completely to semi displacement and remove the keel and mast. Probably need a 70 90 hp.

Also have a look a Charles Atkins "Ninigret hull"

 

Wow Doug you have done a lot of work! Looks great!
I am not looking to get a lot more speed than original hull speed. It will always be a displacement hull but there are displacement hulls of all different hull speeds. I have been on plenty of power trawlers that will not do over 8 knots. I have cruised a 46' Lobster boat design long range, It hull was not a lot different minus the wider keel. It was round chine design and cruised up to 13, most of the time we ran 9/10 for comfort and fuel. That said this boat will be a single digit cruiser! I used a Trim plate on a small sailboat and it really worked well! The key is getting something big enough, mild change in angle. I am using a larger motor than needed (Most likely 30 hp)for 2 reason, I have found the noise escalates considerable and fuel usage are not great over 75% throttle so I want to keep the RPM down, also longevity and control.
I had a really nice Mac 26X, I did custom work for Macgregor yachts for years and I have to say a lot of people put there noses up at Roger Macgregors boats but they work incredibly for what they are designed for. People say oh they are not built strong enough but I am not going to take the boat open ocean cruising. They reason the boats are not built heavy is you can sail all day to a destination and the next morning be back to work in one hour under power. Another thing I will miss is the water ballast trailer weight, Towing that boat I would forget that 26' boat was back there sometimes! I have to admit I WAS A BIG SKEPTIC with the water ballast and was surprised. I have spent weeks getting to destinations such as Mexico and Caribbean powering or sailing along for days and sometimes it not that fun! I can drive to the Keys and throw my boat in and not spend 2 months getting there. I am not looking for a open ocean cruiser, I am designing comfort and easy of use, full standing room, shower, comfortable bed etc. Another big one is the rig, boy is it nice to even have small sails under power sometime! I worked for a rigging shop for 8 years and I look forward to that part of the project. I will get some pictures up later.

 

Here are some pics.

 

Hi Kevin, Now I see what you are after and you are streets ahead of me I really like the lines in your drawing. Cabin work looks great. I don't know what it is but a sailboat hull makes for a very nice looking motorboat.Maybe the almost vertical stem.

So glad you are brighter than me and not cutting up the hull. Don't know about trim tabs on a slow boat could just be more resistance. When motor sailing and the boat is listing trim tabs might be all wrong, Mine are actually a hull extension for more support with 2 guys fishing off the back.

Mine is a lot smaller than yours 17ft more a day picnic boat (Like a Ryds Camping) with 2 single double berths forward so can go fishing with a mate or my wife. Fishing is what the boat is for in semi-sheltered waters.I can trail it up and down the coast drop into a quiet bay and go for short fishing trips. My boat also has a lighter hull than a deep V and my speed range is cruise at 10/12 and top about 13/14 with a 25hp 4 stroke. This is the target and I wait and work patiently on.

I agree a 30 at half throttle nice, quiet and economical.(get a 4 Blade shallow pitch thrust prop good for pushing big boats with small motors.
Idea make your screen low or fold down or a canvas screen Dodger to keep the boom low. A large furling genoa would also be magic with motor running. Remember you want your motor central for motor sailing are you going to use a twin rudder system like the Mac 26. (Not trying to tell you what to do just some idea,s)

 

Bump.Interesting builds.

 

Hello All. I just joined boatdesign.net because of the high quality of information on this forum/thread. I am in the planning stages of converting a MacGregor 21 into a pocket trawler/motor-sailer (depends on whether I add a cut down mast/rigging to the final boat or not... I am designing with mast/sails in mind for now. I am not sure how to best execute the bulwark addition on my project boat, but I am inspired by what members have posted here (James May, your conversion is fabulous, I will want to know more about how you constructed the bulwark... looks great!) 'Looking forward to participating, reading and learning more. Much more. Here, to give you an idea of the conversion I am planning, but roughest 'concept' design... I am detailing the plans at this early stage. I am new to all this.

 

Anybody know any further info on Kevin's conversion. I sent him a pm and so far haven't received a response

 

Hi Chuck, I haven't heard from him. I finished my 17ft conversion, does 18knots with a 25hp.

https://www.youtube.com/channel/UCmPws8z-GsgDxbLBcTNl6sQ

 

Looks great doug. Whats your summary on the project. Would you do it again or purchase a full.planing hull.

 

Kevin hasn't been active on the forum in 2 years.

This sort of conversion can work, to a degree and if you don't ask too much, they're relatively stable. What typically happens is like seen with Doug's conversion. The boat needs to have some plane potential, which his did and if trimmed a few degrees down at the stern, as his does, it'll have enough dynamic interaction to climb up and scoot, which his does as well. The fun comes once it's up and being pushed too hard. The boat will progressively start a few different things, the first is roll and the second if pushed hard enough, longitudinal instability (porpoise). Unless a lot of effort is put into reshaping the under belly of the sailboat hull aft, these things will occur, though with Doug's example, some effort was made and 25 HP keeps it within the acceptable range of motion. I'd like to see more photos of Doug's stern modifications. Any chance you can post some pictures?