Many thanks for all your suggestions, I think I need to read the manual, It may be a few weeks before I can get back to the lake to try anything.

Rob, Ill check that setting but I'm sure that's what its set at as I remember your comments at the time when the last ESC was faulty I'm just hoping this isn't faulty as well.

Doug quite a lot to digest in that lot but there is some hope. The boat did this full power for a short time then drop to half power (switch off disconnect and switch back on) and full power was back. The boat did this a number of times making me think it wasn't a low voltage issue

T33, Thanks for your kind offer of Lipo's, I will PM you to arrange something if that's OK.

dave976 this may be an option when I've had chance to go through things
Many thanks to all

Thanks I have two but both use twin 8 channel receivers and fully independent port and starboard control.
The Speedline model shown has modified speed line jet drives so they don't leak and can be lubricated 😂

My other which is in my Harbour and is a MBD semi scratch build that uses jet drives developed by OceanWorks in Hong Kong with my help. Far superior product 😎

Almost certainly an ESC heat/voltage problem
as others have suggested. You could try a smaller prop to reduce the high current draw (150A ??). Experience suggests many ESCs quote max specs far above what is possible for any length of run time but are usually OK up to 50% capacity.

I like the video T,
She handles well.

Martin555.

If you want to try LiPo I will loan you a pair.
This is 2x 3542 1185kv SK3's on twin 4s Lipo 5000mah in parallel.
My jet drives are 28mm running at 18 to 20k rpm

I wonder what rpm you aim to achieve?

Wrong battery type set is a good bet as Rob says Mike.
The other potential problem is overheating.
Extract from your ESC manual-
"● Protection for overheat: the output power will lessen to half when the temperature reaches 100°C during the operation of ESC.The output power will automatic recover when the temperature is under 100°C."
https://cdn-global-hk.hobbyking.com/media/file/1042643298X416322X14.pdf

With your 19.2V (Nominal!) You are operating at or near the limits of both the motor and the ESC.
Bear in mind that when fully charged your 19.2V battery might well be actually at 22.4V.
(Circa 1.4V or more per cell instead of the nominal 1.2V)
Your meter readings indicate a total of 21V, presumably after a test run?
Thus causing higher current to be drawn and consequent heating.
Do a 'finger tip test' on the next run 😉
Just a thunk 🤕
Cheers, Doug 😎
T33; this ESC does appear to have a battery type selection function.
But strangely NOT via the programming card?
Extract from ESC manual-
"III.The function of electric speed controller for brushless motor
1. Reset to factory default
2. Battery type set (Lipo or NiMh/NiCd)
3. Brake set (yes/no)
4. Threshold set of protection for low voltage (set a protective value for low voltage)"
etc etc

Most likely the ESC low voltage protection setting. Did you buy a programming card for it? Have a look at the instructions and the default settings. This ESC does not have a battery type setting as it is targeted at LiPo but you can set low voltage protection to none https://hobbyking.com/en_us/hobbyking-90a-boat-esc-4a-sbec.html?utm_source=google&utm_medium=cpc&utm_campaign=google_gb_shopping&countrycode=GB&gclid=CjwKCAjwhYOFBhBkEiwASF3KGaBC4n3IvauBeeWJwgAOm5Skgxkwoy6_nrWhrbJcZ6SRd9LnpKQpwRoC9-0QAvD_BwE&gclsrc=aw.ds&___store=en_us

Hi Mike.
Check the ESC is set for NiMh batteries and not LiPo with a voltage cut off ?
I made that mistake on early runs of my Crash Tender.
Rob.