Its a long time since I started this post, but its been winter
I tried a different 400 in the boat, with only slight improvement, so I bought a cheap brushless/ESC combo from Hong Kong - £15 including post. The ESC is forwards only but I figured it was worth a try. When I came to fit it, I couldn't get the 400 out, so I put it into my Deans Fireboat which had a Deans Black 400 on 7.2v and gave reasonable performance, but not really up on the plane.
Just back from trying it out and it is certainly a big improvement running on a 7.4volt 4000mah LiPo about half the hull is out of the water at full throttle in a straight line. The motor and ESC only get warm, not hot, without any water cooling, so I am very pleased. I have another 7.4v LiPo which I plan to try in the Huntsman 31, and will probably get another brushless combo for my Swordsman.
Barrie