hamilton jet query

[kevsabout]

G'Day to all Can anyone tell me what the principles are for selecting an engine to match a hamilton jet propolsion system . Is it rpm or do you need lots of horse power or torque

[PowerTech]

I have only seen hamiltons behind 6LY yanmars they buzz prety high at 3300 RPM making 420 HP not to shaby eather.I know hamilton makes other sized drives I just have not seen any others. this link should educate you on every thing hamiton has to offer.http://www.hamjet.co.nz/

[propellerman]

this is why i am here

[Morgig]

In simple terms the size of the jet is based on engine HP, however just like a propeller, the impeller needs to me matched to the rpm of the gearbox output. If you talk to Hamilton and give them the engine/gearbox combo, they should then be able to give you thrust values. This can then be used in conjunction with a resistance estimate to find the speed of the vessel. This is normally how we work on our vessels (up to around 2x1250 hp installed power)

[FAST FRED]

From a cruisers standpoint the Jets drives are still not worthwhile.

A std prop will push about 20 lbs per hp, a fantastic setup may give 25 at optimum .

The Jets only gove 15 to 17 , so youre paying a lot for the ability to run in shallow water (where the silt & sand degrade the jet very quickly).

IF you MUST have them for your operation the tradeoffs are acceptable ,

FAST FRED

[Morgig]

It's not all about the ability to run shallow water and speed. Waterjet boats have better manoeuvrability, more damage tolerance and greater safety when working with people in the water. Sure if you just want out and out speed go for an outdrive or surface prop, but there are other factors to consider

[Karl2]

Apart from the shallow water benefits, potential swimmer safety benefits, manoeuvrability (percived or real ??) there are high speed benefits with jet propulsion.

Different companies/naval architects etc. use different terminology for efficiency ("Net Trust Efficiency", "Transport Efficiency", etc.) but.... A jet compared to conventional inclined shaft is more efficient at speeds over 35 knots - higher speed = greater gap. At speeds below 35 knots shafts are more efficient - slower speed = greater gap.

Not even sure what Fast Fred means with "A std prop will push about 20 lbs per hp, a fantastic setup may give 25 at optimum". Any statement re this has to be qualified with a reference to boat speed. If it is a planing boat and 20 lbs of displacement per hp I would call that a non-performer and 25 lbs per hp a poor performer. It is very common that you get very good performance at 30 lbs per hp (Unless Fred is talking about a inboard boat in the 40+ knot speed range).

Karl

[cyclops]

I think the huge ferry catamaran that runs between Bar Harbour, Maine & Canada, does 40 to 45 all the time. They have to be way over 25#/ hp.

[Willallison]

I believe Fast Fred is talking about 25 lbs thrust per hp - nothing to do with the boats displacement.
My understanding is that up to 30-35 knots shafts are most efficient (lets leave sterndrives out of this for now). Then water jets up to about 40-45 knots. Beyond that you are well into the preserve of surface piercing drives.
Most fast (30-odd knots) ferries run waterjets for exactly that reason - they are the most efficient at that speed.
The problem for recreational users is that we don't tend to operate withing such finite speed ranges. When it's calm we go faster, when it's slow we go slower - and it's when we slow down that waterjets lose out the most. They are (generally) horribly inefficient at semi-displacement and slower planing speeds.
But if you're going to run over a swimmer or two then nothing elso comes close !!!

[Morgig]

There are very real increases in manoeuvrability over prop. leaving aside the new 3d joysticks, having twin waterjets means you can vector your thrust, meaning you can go fwds, backwards sideways and rotate without bow thrusters. You can also have the engine operating at high load and control speed using the buckets, giving you better response. All this is why alot of new pilot boat are having waterjets fitted.

[cyclops]

Do I feel the fuel bill going thru the roof on constant speed jets?

[Willallison]

Depends how fast you go....

[Morgig]

They also use this to match the speed of a set of waves in bad weather and could be said to be like changing down a gear in a car to get more torque going round a corner.

[RANCHI OTTO]

The main items are rpm and power.
Normally to satisfy the rpm limit you will need a gear box.

Normally the steps are as follows:
(1) select the engine power according to a speed vs power forecast for waterjet

(2) select an engine with the required power

(3) select a waterjet model for the power and rpm

(4) if you need gear box, select the standard "i" offered from the gear box manufacturer.

[tom kane]

Try www. hamjet.co.nz should get answers there.