Too slow unless steaming with sea or tide?

Question

i have an air cooled lister in my boat , she is 21 feet and weighs about 2 ton odd, but only steams at about 5 to 6 mph(bout 4 knots) could the gearbox be slipping or maybe wrong prop. how do you adjust gearbox or how do i find right prop, any information is much appreciated, any ideas mactheboat?

Answer 1

matchbox alluded to what might be the problem.a displacement hull.......and if you're lugging around a good old Lister you're NOT a planning hull.....has a MAX speed of square root of the waterline times about 1.15 in knots...which gives a MAX of about 6 knots for you.....

and have you looked at the prop? even the slightest fouling will reduce the efficiency, and a half dozen barnacles will really knock you back.......

Answer 2

I assume from your previous posts that your engine has a standard Lister ,mechanical gearbox, which are usually trouble free - but can be adjusted.(see hand book- its a long time since I did one!) If it goes into gear with a satisfying clunk, that should not be your problem. Another sign of the box needing adjustment is if it wont stay in gear.

When you drive the engine at full revs,on load (in gear) do you get any black smoke at the exhaust ? If not you may need a bit more pitch on your prop.  The size of your current prop will (should !) be stamped on the boss, below the nut that holds the prop on! Some small props have it stamped on the boss,between the blades.

A boat the size of yours,due to its waterline length is never going to do much better than 6-7 knots without the benefit of a lot more h.p.(50+) but it is nice to get the full value of what you have. Are there any similar boats in your area,that you could borrow somebodys spare ,to see if it makes a difference.

Answer 3

need more info  motor hp  prop size   tach at W.O.T.  if it tachs to high  changs the prop  bigger pitch small diameter but i cant tell you for sure ned more info

Answer 4

First of all, you need to know what is the hull speed of your boat.

Answer 5

Aircooled means it's SMALL, less than 10hp so it won't push a non planing hull at more than 6kts.