Following the sea (lake) trial there have been three props tested on the Vipermax and I need to make a choice, I think I know what I want but just after some comments. For the sake of clarity, we are talking about a Vipermax 5.8m, single dive console plus two extra single jockey seats, A frame, and a 4hp aux, all being pushed along by an XL shaft Etec 150hp, usually with just one or two people on board plus up to 150L fuel.
A lot of folks especially on the Etec Owners Group say that you should prop an Etec to run in the top half of the WOT range, from what I read on tinternet this is to keep the engine revving freely and not labouring.
Roy Bishop who is building the boat says he doesn't agree with this and his point is that the Etec 150 is a very unstressed motor in this application, being the lowest HP version of the 2.6L V6 Etec block, and the largest HP rating for the 5.8 hull, his view is that it can be propped to have long legs for cruising at lower rpm.
Obviously the two arguments are opposite ends of the scale ... so what do I do?
The sea trial tested three props, a Raker 20", a Viper 21" and a Viper 22". Results for WOT were:-
Raker 20" 54mph (47kt) @ 5400rpm
Viper 21" 52mph (45kt) @ 5100-5200rpm
Viper 22" maxed at 5000rpm (didn't ask what speed) so have discounted that.
The Etec 150 has a WOT range of 4850-5850rpm and rated power comes up at 5350 according to the spec sheet which is right in the middle of the WOT range.
I am going to take the Raker anyway as a) it seems suited to the boat, b) it is a second hand one that Roy is offering at a good price and c) I want a spare prop just in case of accidents and cheap is good
So I can have the choice of any prop in the range for the "new" one... the obvious candidates being the Viper in 19", 20" or 21"
I think a 19 will be too revvy and going on the 300rpm per inch rule, would probably be hitting somewhere up around 5800rpm which I don't really want, I have never been a fan of screaming the nuts off any engine as that is usually when conrods start putting in an appearance and we don't want that... so that leaves a choice between 20 or 21. Absolute max speed / rpm is not something I use all that much - and with a big engine on a small boat it is pretty well going to get up and go like sh^t off a shovel whatever goes on it.
I'm thinking a 20 might be a better bet but I have one unanswered question; which one is likely to give the better fuel economy cruising long distance at say 25 to 30 knots? I'm not really sure whether, at any given speed, dropping the revs but loading the engine a little more will increase or decrease fuel consumption?
He did some economy checks (bit difficult as they kept running out of lake...) but observed (I think with the 21" Viper) that the economy at 30mph (26kt) was 20-21lph and best appeared to be around 27mph (23.5kt) at around 17lph which is pretty good at about 0.72L/nm, if it manages that on a regular basis I will be very pleased
I don't suppose it is going to make a huge amount of difference but since I have the choice, I'd just like some opinions please...