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Thread: 20 HP Mercury and 20 HP Tohatsu Racing Propeller

  1. #11
    Team Member GoFast Cracker Bay's Avatar
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    The Tohatsu and Mercury 20's are remarkably similar. I wonder if Tohatsu is making them for Mercury or if they started with the same deisgn and made it theirs.
    Jim Keith
    CrackerBayThundercats.com

  2. #12
    Team Member Mini Max's Avatar
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    Quote Originally Posted by GoFast Cracker Bay View Post
    The Tohatsu and Mercury 20's are remarkably similar. I wonder if Tohatsu is making them for Mercury or if they started with the same deisgn and made it theirs.
    All Mercury badge engines 30 hp and under are manufactured by Tohatsu.
    www.Compmillennia.com Builder of Lightspeed Catamarans
    Likes GoFast Cracker Bay liked this post

  3. #13
    Administrator Ron Hill's Avatar
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    Default 20 HP 4 Stroke Props for Signature Propellers

    Made some 15, 16 and 17. Reported speeds were 44 with the 17.
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    Team Member Mini Max's Avatar
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    Quote Originally Posted by Ron Hill View Post
    Made some 15, 16 and 17. Reported speeds were 44 with the 17.
    True, but it was a one time, first try with a 480 lb motor, boat and driver. (I am 200) No Tach fitted.

    The Carolina Virginia Racing Association has purchased a 4 stroke Tohatsu and commissioned John Runne to build a purpose built runabout. It will be raced and used in their driver school. John has all of the Hill Marine props and will give them a fair test. We think 50 mph is a reasonable expectation.
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    Team Member Mini Max's Avatar
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    The four stroke engine is legal to race 15SSR and BSR APBA Stock Outboard. 2023 there were 30 drivers scoring points in 15SSR and 0 drivers scoring points in BSR, although 63 drivers scored points in BSH. I assume the wisdom is that the 20 four stroke can race with 15SSR provisionally but when numbers justify then BSR would race as a four stroke 20 class.
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    What rpm were you running and prop diameter? In my opinion we need to build new boats to match the engine. For a utility I’d look closely at the GT15, 30 and Dillon boats for design baseline. For a hydro you need a longer, wider, deeper boat with rear shoes to decrease hull drag. Would also go to a boat mounted water pickup to allow the motor to be jacked up as high as you want. I’m thinking bigger diameter props and keep adding pitch til you hit the rev limiter. Use an adjustable jack plate or transom to make height changes quickly to figure this out quickly. We ran 40+ with 20 Merc’s on tiny titans and flat bottom river racers back in the 70’s and 80’s as kids. The speed is there, just have to unlock it. This is an EFI engine which is totally opposite of a carbed engine in that the carbed motor you can under prop it and spin the hell out of the wheel to get speed out of it. As a point of reference I had a 16 ft crestliner utility with an Evinrude 50 Etec on the back. I quickly found out I could not under prop it, hit the rev limiter right away. I kept putting more pitch to it til it ran out of steam and that was 21p and 46.4 mph. Respectable but given the power to weight ratio should have been able to run 52-55 which I believe could have been achieved with a low water pickup nose cone. So in summary here’s what I would concentrate on; prop match to EFI and new hull with the least amount of drag possible. For a hydro I would use a bigger version of the HARM J boat (same boat that HRL novice drivers built last winter, Karelsen/Kelsen design similar to the 70’s early 80’s boat Ron posted on BRF FB). Wider cockpit, the extended nose on the boat would accommodate hotfoot for sit down ride. Make the seat movable for/aft to help find the balance of the boat. So 10’8”, 40” wide air trap, 2.5 degrees AOA, etc. I see that a group headed by Dewald has decided to adapt a racing foot to the 20 4 stroke. I would say the ROI is not good considering the gains that could be made with a low water pickup nose cone or remote water pickup for a lot less money. Speeds were 51 on the utility and 48 on a hydro. I am certainly not a proponent for a specialized foot and perpetuating the current status quo which does nothing but hinder growth and opportunities in boat racing.

  7. #17
    Team Member Mini Max's Avatar
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    Engine height I set with the same APBA Brown height checker I use on my CSR Yamato. CL of prop shaft 3/4" below the bottom. Up to 85 mph the round nose mercury gear case on the 300 hp Mercury's I use on some cats I build is better than the pointy nose low water pickup "Sportmaster" There isn't going to be any gain from spending a lot more money to modify the 4 stroke with parts no longer made.

    Yes you might want to use the left hand rotation gear case because of all the props on dusty shelves but the European prop makers LAK and Winrace provided me thru hub exhaust props from the GT-15 classes and Ron Hill sent me over the hub exhaust RH props he made.

    The numbers I heard with the 20 four stroke modified with a "A" racing lower unit were the runabout 44 mph and hydro 51 mph. 44 was the speed the first time I tried the box stock Tohatsu with either the European props or the ones Ron made. I come at this as an average guy, not from a racing family. It took me a year to find a good C Yamato 102 and a refurbished D Tohatsu 50. To get the 20 took 15 minutes, the time it took to drive to the marina and take my pick of the ten 20 hp Tohatsu's in stock.
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  8. #18
    Team Member Mini Max's Avatar
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    The c/l of the prop shaft 3/4" below the bottom is the same as the UIM GT-15 and GT-30 classes and the APBA stock runabout classes. The Tohatsu has a electronic guardian mode for overheating and no need for modifying the engine for low water pickup has become evident. There are many "tricks" we use to get water to a stock engine that are non invasive but why? The engine cools without messing with it so far.

    By buddy Harry Schoell is also building me a prototype prop. He is 80 and we are working to get him to the Top of Michigan Marathon 2024. Before Todd Anderson provided me my first props from LAK, Harry designed a prop for the four stroke 20 that he is building in his garage. Props and boats. Boats and props. My DSR came from a sketch he made of his CSR he built in the 60's.

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    Ok, so no cooling issues so nothing to do there. If you are going to run that boat you need to add a pad to the bottom. I would say 1” thick by 8” wide and 2’ long blended into the boat. It might not handle well with this. Other option is to add steps. You have to much drag at this point. Really need a Dillon boat or a smaller version of a delta, critch or voodo. In fact, I saw an 11’ delta on kijiji in the summer, they are really uncommon. A Zorkan might work also. I think this is a boat issue at this point, need something to handle the high c of g and be able to set the balance. My dad had a 1973 14’ Springbok with a 1985 Mariner 25 hp that would do 42 mph with just me in it. That whole outfit weighed 292 lbs. The GT 30 kilo record is almost 60. So there is a lot on the table with these 4 stroke engines. Might want to check with the TBoat guys on the High Performance Boating Canada website.

  10. #20
    Team Member Mini Max's Avatar
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    APBA stock outboard has a very few boat rules but one is no more than a 5/8" longitudinal step in the hull bottom and no transverse step. Tim Kurcz bought me the Dillon plans and these photos are of his Ropp/Kurcz design DSR "Summer Daze" a high point champion. His pad has the outside edges beveled. This makes the boat track better and gets the thickness to within the rules.

    My friends had a rule when bar crawling which was "go ugly early" then the pressure was off to find someone better to take home that night. I never bar crawled but I have had a lot of boats and I suppose "go ugly early" got me boat riding first, and now the pressure is off to find something better.

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