Model Boat Mayhem
The Shipyard ( Dry Dock ): Builds & Questions => Pleasure boats, Sports, Race, Power and Leisure Boats: => Topic started by: SimonCornes on July 29, 2025, 06:52:11 pm
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Can someone tell me what motor should be fitted to this outboard please? I have one attached to an F1 tunnel hull and it used to have an Align RCM BL700MX of 510 Kv which I am told can be used in 700 type RC helicopters with 12S batteries!! I don’t know whether that’s the original motor and even if a motor that big is required? There doesn’t seem to be any water cooling intake in the skeg. Any suggestions would be appreciated. Thanks
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Found this by Googling which may indicate what the original motor was?
GRAUPNER GTX-800 SPEEDLINE OUTBOARD MOTOR WITH SPEED 820 TURBO BB RACE MOTOR INSTALLED. The motor housing is hard plastic while the lower half is all metal. Could be Aluminum. The prop is turned by a...from
If you Google that motor it should give you the specification. Replacement motor will depend on how much space you have. Brushless may be a better option?
How big is the boat and what size prop?
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I have exactly that - a Graupner GTX800 with its Speed 820 BB motor installed.
Picked it up for a song many years ago. Here's the spec :-
http://jethobby.com/_shop/product/product_detail.php?id=10864
Hell of a motor, runs on 20Volts, no load rpm 26,000, stall current 166Amps !
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Found this by Googling which may indicate what the original motor was?
GRAUPNER GTX-800 SPEEDLINE OUTBOARD MOTOR WITH SPEED 820 TURBO BB RACE MOTOR INSTALLED. The motor housing is hard plastic while the lower half is all metal. Could be Aluminum. The prop is turned by a...from
If you Google that motor it should give you the specification. Replacement motor will depend on how much space you have. Brushless may be a better option?
How big is the boat and what size prop?
I’ve now left the hull somewhere I can’t measure it now but I’d say it’s about 600mm or thereabouts and unfortunately I can’t tell you the prop diameter but it looks ‘right’ as in not too big or two small but I don’t know what pitch. Think it’s plastic - probably as supplied by Graupner originally?
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I have exactly that - a Graupner GTX800 with its Speed 820 BB motor installed.
Picked it up for a song many years ago. Here's the spec :-
http://jethobby.com/_shop/product/product_detail.php?id=10864 (http://jethobby.com/_shop/product/product_detail.php?id=10864)
Hell of a motor, runs on 20Volts, no load rpm 26,000, stall current 166Amps !
Ha! You might have been lucky. There was a chap selling one on eBay a couple of years ago for a sensible price (£15 plus postage) but that sale is no doubt long ago completed!! I don’t know if Graupner still sell such motors? Probably worth checking with Prestwich or Cornwall Model Boats?
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Video: https://youtu.be/uIhMFHYbE24?si=aVCzkAiwkSGG3KuV
(https://i.postimg.cc/p9hqBPMk/Screenshot-2025-07-29-203106.jpg) (https://postimg.cc/p9hqBPMk) (https://i.postimg.cc/kV0w1LnC/Screenshot-2025-07-29-203127.jpg) (https://postimg.cc/kV0w1LnC) (https://i.postimg.cc/Sntg8J3J/Screenshot-2025-07-29-203152.jpg) (https://postimg.cc/Sntg8J3J) (https://i.postimg.cc/dh0WL8Wb/Screenshot-2025-07-29-203226.jpg) (https://postimg.cc/dh0WL8Wb) (https://i.postimg.cc/7f9sHBN1/Screenshot-2025-07-29-203247.jpg) (https://postimg.cc/7f9sHBN1)
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Video: https://youtu.be/uIhMFHYbE24?si=aVCzkAiwkSGG3KuV (https://youtu.be/uIhMFHYbE24?si=aVCzkAiwkSGG3KuV)
Grief they’re fast!!! Wonder if that’s just a 5S Lipo?
Interesting photos below, any idea what the gold can motor is ?
(https://i.postimg.cc/p9hqBPMk/Screenshot-2025-07-29-203106.jpg) (https://postimg.cc/p9hqBPMk) (https://i.postimg.cc/kV0w1LnC/Screenshot-2025-07-29-203127.jpg) (https://postimg.cc/kV0w1LnC) (https://i.postimg.cc/Sntg8J3J/Screenshot-2025-07-29-203152.jpg) (https://postimg.cc/Sntg8J3J) (https://i.postimg.cc/dh0WL8Wb/Screenshot-2025-07-29-203226.jpg) (https://postimg.cc/dh0WL8Wb) (https://i.postimg.cc/7f9sHBN1/Screenshot-2025-07-29-203247.jpg) (https://postimg.cc/7f9sHBN1)
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Motor in the last photo is an inrunner brushless. They are usually used in fast/raceboats and would be a good choice as they provide high revs and wouldn't need a very high voltage battery. That 12S you mentioned originally is just crazy!
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Motor in the last photo is an inrunner brushless. They are usually used in fast/raceboats and would be a good choice as they provide high revs and wouldn't need a very high voltage battery. That 12S you mentioned originally is just crazy!
I’m thinking 4 or 5S should be the max but could you suggest a suitable brushless in runner please?
Thanks
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Yes that is about the voltage I was thinking about.
As regards size as a starting point you need to measure how much room you have to determine what diameter and length of motor will go in. Advantage with an inrunner is that the outside casing doesn't turn. The type of coupling will also have to be determined as the length will have to be accommodated.
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Yes that is about the voltage I was thinking about.
As regards size as a starting point you need to measure how much room you have to determine what diameter and length of motor will go in. Advantage with an inrunner is that the outside casing doesn't turn. The type of coupling will also have to be determined as the length will have to be accommodated.
Well the outdrive has a black nylon X45 prop which I imagine is Graupner. The coupling had previously had a brushless motor with a 5 or maybe 5.5mm diameter shaft. As for space inside the plastic cover, I’d say there’s 95mm between the lower point to which the motor bolts and the top of the inside of the cover with maybe a 45mm max diameter. I have no idea what motors would fit that space. There is no water cooling, just vents on the top of the plastic cover. I’m assuming that an 800 size motor can is the maximum that would fit ?
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OK. I think from my previous Googling that the motor shaft diameter was 5mm which is a standard size.
For decent performance from my Fairey sports cruisers (single hull) I aim for around 12k. rpm unloaded as you need around 10k. rpm unloaded to run well on the plane though in reality I'm probably running at lower than that. So as yours is a race boat type hull I'd base it on say 20k. rpm unloaded, unless you want manic performance which may bring problems as regards the motor getting too hot?
So at 20k. rpm and running at 4S (14.8volts) you are looking at a kV of around 1350 which isn't that high so no problem there.
If you look at the Noahs Ark RC website they do a number of suitable motors. One that fits the bill is the SSS 3660/2070kV which will give you a few more revs.
I don't watercool my builds as I don't run at continous high speeds and vary the speeds. If you intend to run laps at continuous high speed you might have a problem. Trouble is outboards are difficult to watercool as ideally the water inlet needs to be behind the prop and it all looks a bit messy. It can work with the inlet on the hull though, especially on a fast model. Good thing with inrunner motors is you can fit a water jacket.
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Thanks Chris,
Any thoughts on suitable ESC's ? I looked at the Noahs Ark site and came up with a Flycolor Fly Monster ESC-90A but that is designed to be water cooled. I have no knowledge of this sort of ESC!!
Simon
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That would do, you don't have to connect up the water-cooling and if you need to after trying you have the option then.
I mainly use HobbyWing Quicrun which are fan cooled (though with my builds I remove the fan) and the HobbyWing Seaking which is the water-cooled version. I haven't run one of my builds yet using one of those but the intention isn't to water-cool.
Whichever you choose I recommend getting the programme card for it as it makes setting up the ESC so much easier.
I get my HobbyWing ESC from Wheelspin Models.
Here's a shot of my Fairey Huntress which has a stern-drive fitted and has a Quicrun ESC with the fan removed.
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Thank you Chris, I'll have a look at those. Nice boat by the way!