Glen-L.com

Hi,

I'm looking at the study plans for the Bandido and it frequently mentions the use of twin engines (or one HUGE single inboard). I don't want to go on any poker runs or even do over 60MPH (I ride motorcycles and rarely push a bike over 80MPH). So, I'm thinking about twin 115HP outboards. Would those be too anemic for a Bandido (I'm considering the lighter weight, open cockpit version)? I see most saying that 400HP (or twin 200/225) are minimum. I have two neighbouring islands here that are 10-15NM away and I will RARELY go 30NM out fishing. Since gas is $5/gal on island I'm trying to keep the fuel efficiency up as well.

http://maps.google.com/maps?f=q&source= ... 12&iwloc=A

you might have to zoom out twice to see the other islands

I can give you a "rule of thumb" formula that I use when rigging powerboats to illustrate to prospective owners why I suggest "horsepower X" based on "their" expectation of how they want the rig to perform.

What you will first need to do however, is estimate the actual rigged weight of the boat (including engine(s), gas, gear, batteries, passengers etc) - when it will be on the water under "normal" operating conditions.

That can be a little difficult on a hull that is not completed, but plans or working info can often give you a (possible) hull weight. To be safe add 10-15% to that and then estimate the rest (A 115 horse 4 stroke or DFI will weigh in at about 400 pounds etc).

So just for the purpose of an example the hull weight of the Bandido (according the the Glen-L site) is estimated to be 2700 lbs (wood version), add 800 pounds for motors, 200 pounds for batteries and other rigging (cables, controls, steering etc), upwards of 400 pounds for fuel (which may be conservative since a pair of 115's could burn 20+ gallons per hour at wide open throttle), and allow 500 pounds for passengers and their "junk for the day" = 4600 + 10% "just incase" = 5060 pounds "loaded" (which is close to fully loaded now since the displacement weight is on 5350)

So with a weight of 5060 pounds we can "estimate" how fast this rig would run with a pair of 115 horse outboards.

(I didn't create this math, most of it is based on theories from a math "dude" name Froude who came up with a bunch of hydrodynamic formula's - you can google him if you are really interested)

Anyhow, you divide the actual displacment (loaded weight) by the shaft horsepower of all motors =

5060/230 = 22 (this is called your power to weight ratio)

You then divide a "hull factor" (190 in the case of a high speed performance boat - (planing hulls have "hull factors" of between 150-190 and the number isn't static but using 150 for a runabout style hull and 190 for a bass boat/cigarette style hull is close enough)) by the square root of the power to weight ratio =

190/square root of 22 = 190/4.69 = 40.51 (estimated speed in Knots)

40.51 x 1.15 = 46.6 MPH is the theoretical top speed you could get with that hull/motor(s) combination IF the rig was operated at sea level, with an air temperature of 70 degrees and a relative humidity of 30% (ya I know )

In "real life", if you reduce that "theoretical" speed by 10-15% (to account for atmospherics and less than "perfectly" tuned motor(s)) - you get a pretty good idea of how the rig will perform.

In this case, probably 40 mph would be a "reasonable" expectation.

I do have another formula that will tell "how many horses you need" if you want to achieve "X" speed, which is what I use far more often - most new boat owners have a speed in mind that they want to get and then rig from there.....

_________________
Graham

Yes, Plywood is "real" wood

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it

galamb,
I just wanted to thank you for your very informative reply. I am in the planning phase for my Bandido and was wondering the same thing. I want different use though. I will be using mine mostly on lakes and rivers for wake boarding / tubing, but when I am not towing anyone I want to go fast - very fast. I will be using the high-tech fiberglass version. What I am wondering is at what point during the build do I need to decide which type of drivetrain I am going to use. I also need to find out which type will give me the best wake for wake boarding.

capnmike,
how is your build going? I am in the planning phase for mine (explained in previous post).