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Thread: Runabout Outboard Size Help

  1. #1
    Join Date
    Apr 2021
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    Lightbulb Runabout Outboard Size Help

    Hi Guys, I'm new to the forum and to the wooden boat world in general but very enthusiastic. I came across a 1950's wooden runabout in an old garage which I obviously had to buy on the spot. It most likely hasn't seen the water in 30 years but has very little rot. I build furniture and have a nice woodshop so I figured I'd give the rebuild a try. I'm hoping to keep it as original as possible. There's no model number, or markings...I don't even know the make but I was hoping you guys could help me determine which outboard would fit best. It is 13.5' from transom to bow, transom height is 16.25" and made of mahogany and teak; 4 people can easily lift it and flip it over (which we just did as I'm now working on the hull). I was thinking 30-40 HP short shaft (15") but want to make sure this will get me on plane (~15-20mph) but won't be too heavy for the transom. Unfortunately it looks like these outboards are on back order but I want to make sure to invest in the right one. I've attached a few pictures, thank you in advanced for you help!
    IMG_20200609_171516.jpgIMG_20200609_171421.jpgIMG_20200609_171551.jpgIMG_20200614_174250.jpg

  2. #2
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    Default Re: Runabout Outboard Size Help

    In it's day, a 25 was a big motor. There was no 40 hp until 1956, and the shape of boats changed quickly, round bilges just about disappearing on planing boats.
    Modern motors are indeed heavier, but have power trim and better props .
    I dunno if I'd put a 40 on her.
    A new 4 cycle 30 I think would be perfect.
    Hope you share the resto.
    There is a buncha amateurish lookin stuff there,I mean corny/campy .
    Lets see what the bottom sheathing is ? An old stripper should take being sheathed nicely.
    Simple things, well, not so simple really, like fairing the bottom, sharpening the trailing edge radius, power trim, can add 20 percent to speed as good as 10/15 more horsepower.
    I do leaps and bounds to get my own 13' , 25hp speeder to be faster than most 40/50 hp runabouts,fer instance.
    If you only seek a comfy 20 mph, a flatter wheel (prop) may be all you really need with a 20 hp.
    I see she has a new trailer, be sure the bunks carry aft past the transom a few inches.
    bruce
    Last edited by wizbang 13; 04-30-2021 at 09:23 AM.

  3. #3
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    Apr 2021
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    Default Re: Runabout Outboard Size Help

    Thanks for the input @wizbang 13!
    There is a pretty big weight jump from 30 to 40 HP so this really helps.
    The bottom has a layer of fiberglass which was obviously applied retroactively (and not done well). I have already removed the antifouling bottom paint and plan on repairing and fairing the bottom smooth, a new layer of epoxy resin and antifouling paint. I was planning on keeping the fiberglass to provide some rigidity to the boat.
    The trailer is only a temporary trailer to relocate the boat to my garage workshop. The original one was completely rusted out.

  4. #4
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    Default Re: Runabout Outboard Size Help

    One may want to check if the glass job was done with polyester or epoxy...or even fiberglass for that matter.
    If it peels off the boat goin fast ...thats no fun.
    If the boat does not stay in the water, there is no need for anti foul.
    It's nice you are investing in a new mill, she looks like a beast to row.

  5. #5
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    Default Re: Runabout Outboard Size Help

    I would second what Bruce (Wizbang) said. I would never put 40 hp on that runabout, and similar to Bruce think that you you could probably get the performance you are looking for with even less than 30.
    While growing up I had this 136 OldTown runabout (very very similar) and had a 25 hp on it. It was more than enough, and we even did a lot of water skiing behind it. We never tried more than the stock prop and I think it would do about 25 knots (27-28ish mph).
    Attached Images Attached Images

  6. #6
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    Mar 2021
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    Default Re: Runabout Outboard Size Help

    I agree with whizbang on his observations and recommendations. I started messing with small outboards when the most common size motor was 10hp and the largest commonly available was 25hp. Mercury and a couple others actually had more power than they were rated at. I expect that this boat could take a 50hp if handled safely but the extra power is just not needed for what you want. A 25 is adequate for delivering a great speedy ride while a 30 might be a bit nicer. Its good to study the weight versus stated power versus displacement in making the best choice of a motor. My latest was a 13' 2" runabout with an old Johnson 25 mounted and top speed with no modifications was about 32mph. Such a small boat with the driver so closely connected to the water feels pretty fast at that speed.P1010005.jpg This boat has a 1953 Merc MK20 on the transom but it was never used because it had only been in fresh water and a Johnson 25 was actually used.

    hope the photo comes through as I've never use this computer setup. The original 12' boat had a seconf 10hp motor added and is doing about 39mph in the photo. The driver was a bit younger then and had hair.
    Attached Images Attached Images
    Last edited by Sandlapper; 04-30-2021 at 11:07 AM.

  7. #7
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    Default Re: Runabout Outboard Size Help

    If it is of any assistance to you **, Transport Canada's "Construction Standards for Small Craft" TP1332E 4.3.3.1.2c Recommended Maximum Power Calculation for a vessel with midship deadrise angle greater than or equal to 5 degrees, with remote steering, yields a max power recommendation for a 13.5-ft LOA x 4.25-ft transom beam hull of 25 horsepower.


    ** (I say this because I realize that the opinions of naval architects and the rulings of national compliance agencies - in this case Transport Canada, which is pretty much compliant with ISO standards) - are not held in very high esteem in this Forum)
    Hope for the best, but plan for the worst.

  8. #8
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    Default Re: Runabout Outboard Size Help

    I've seen that with the newer four strokes there is a huge difference in size and weight between a 20 and a 25hp.
    I think you would do just fine with a 20hp on there. Maybe less.
    It's not that big of a boat...
    Each step up in horsepower make a big increase in cost too.

  9. #9
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    Default Re: Runabout Outboard Size Help

    I have run the same 5 metre runabout over 15 years with a 2 stroke 40 hp Yam, a newish 4 stroke 40 Yam, and a 30 hp 4 stroke Honda. the perceptible difference is the 40s are 25 kgs heavier, 3 cyl instead of 2, but have a top speed , one person no dogs or camping gear of 35 kts , two stroke expensive and noisy and stinky to run, and a bit faster, the newer 4 stroke much quieter, almost half the running costs , partly because of no oil, The 30hp honda was almost as fast but had to work hard and so cost the same in gas per hour. and had to run at full crack to get there. less relaxing and more noisy and stressful. The weight difference is only a tank of gas, apart from grunting it on and off the transom for periodic transom refurbs. still, I'm not lifting it on my own like twenty years ago.

    To summarise go for the bigger motor, weight difference is negligible , resale value is better, the motor works less hard, fuel consumption difference not a factor, you'll enjoy that bit of extra, maybe even need that bit of extra in coastal and estuary or tide rip situations.
    And with 50 kgs of dog 60 kgs of passenger camping kit another 30 kgs at least, another 40 kgs fuel....no contest

    Tootling around on inland waterways, maybe two of you and day bag of beer and sandwiches.....I'd listen to the opinions of the 20 horses crowd. go for the 20 or 25.
    my next project in that area I'm going electric.
    Bilious, choleric, sanguine and phlegmatic

  10. #10
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    Default Re: Runabout Outboard Size Help

    he newer 4 stroke much quieter, almost half the running costs , partly because of no oil,
    True enough, no oil while running.

    But you do need to change the oil annually or every hundred hours.
    A 30-40 takes two quarts, which costs 25 bucks. Plus the filter is about 10 bucks. So lets say $35 bucks a season for oil

    Can figure it by hours or gallons...but le;s make it simple. Say you use the boat ten times: Add $3.50 per day on the water for oil.

    Or, $35 bucks buys a gallon-and-a-half of two stroke oil. That's 12 pints--good for about 70 gallons of gas burned. What's that 30 burn at 3/4 throttle? Maybe 2 gph?

    At any rate, no oil while running. But oil is a cost of ownership nonetheless.

    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

  11. #11
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    Default Re: Runabout Outboard Size Help

    the OP wish of 15 to 20 mph is reasonable.
    Pushing a boat like this to 35 or 40 mph is way beyond what it was most likely designed for from the beginning.

  12. #12
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    Default Re: Runabout Outboard Size Help

    Thanks for all the input guys! I'll go with a 25 HP. It appears to be really hard to find a 25 HP short shaft outboard these days. With the 16.25" transom, do ya'll think I could get away with a 20" shaft and mount it a little higher? I obviously want to avoid cavitation and get the ideal outboard but a new 25 HP 15" shaft outboard is at least 5 months out on back order. Thanks again!

  13. #13
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    Default Re: Runabout Outboard Size Help

    I do note - with some amusement - that the replies have fallen roughly into two camps: there's the "how much is enough" crowd, and the "how much will it take" group.
    Hope for the best, but plan for the worst.

  14. #14
    Join Date
    Apr 2012
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    Default Re: Runabout Outboard Size Help

    Don't forget about Suzuki when shopping. I bought a 20 hp remote control version and so far I'm happy with it. It came with the shifting/throttle controls and harness. Not as popular as other brands here but very popular elsewhere and they seem to put more effort into fuel economy. Loads of power for 20 hp.

  15. #15
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    Default Re: Runabout Outboard Size Help

    Try these guys if you want a Suzuki.
    I bought from them with zero problems. Shipped right to my house.
    https://www.porta-bote.com/suzuki/

  16. #16
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    Default Re: Runabout Outboard Size Help

    Look for/ ask for/ research the rigging details for each engine. The mounting bracket will have three or four mounting holes, so that a 20-inch shaft motor may be mounted for a 20-inch transom ( using the top mounting hole) or a 16- or 17- inch transom using the lowest mounting hole. Or, in between. The Suzuki shown below has a four hole mounting bracket.


    Screen Shot 2021-05-06 at 11.14.19 AM.jpg

    Mount the engine so that the anti-ventilation plate is even with the bottom, or just below. Do not worry about the bracket actually resting on the top of the transom, as shown in this image. That is not a criteria for mounting; the anti-vent plate height is. The bolts will be more than adequate to support your engine.

    Screen Shot 2021-05-06 at 11.12.54 AM.jpg


    Also know that while the industry has "standard" shaft lengths of S(15"), L(20") XL (25") XXL(30") these are nominal numbers. Here are the lengths for a Yamaha 25, from the owner's manual. I googled it. Its "15" is actually 16.7-inches.

    Screen Shot 2021-05-06 at 11.23.22 AM.jpg

    So, for a Yammy, on your transom, only a short shaft and only mounted using the top hole in the bracket will work. Check for each make you are considering.

    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

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