Accomodating a Jet Drive

[AuxiliaryComms]

Hello all, I'm not sure if this is the most appropriate spot for my questions or if a more design oriented section of the board would do better but here goes.

I am conceptualizing a design for a multi-purpose boat for my own use. Without going through all the details, I am looking at about 55 to 65 feet, designed primarily as a liveaboard with the ability to do towing, and search and rescue operations (space on deck and in cabins to treat injured people) and with a limited fire-fighting capability. I'm expecting to use a generator set or two to drive two electric motors and provide electricity to the rest of the boat. At this phase I think I want those motors to be running jets.

I'm thinking of jets for several reasons. They will allow me to get in closer to shore than large props would. They are safer for people in the water, and if what I've seen is right, they will offer a lot of maneuverability if paired with the right controls.

While I welcome opinions on the above my main concern right now is with the hull design. What considerations do I need to make to accommodate that jet system. Does there need to be a flat section for the jets to mount through or can they be mounted to angled plate. What about curves. This is my first hull so I can come up with more questions or more information if you need it.

Thanks for any help you can give me.

[marshmat]

Hi Jason,

I've been doing some research on the newer jet pump designs as well, albeit for a rather different style of craft.

Regarding the pump itself, it seems that a lot of the common wisdom about jets being inefficient at lower speeds is not really true. It is true that a small diameter, high RPM jet will waste a lot of power at lower speeds. If you contact HamiltonJet and ask them, as I did, you'll find that they can recommend (and supply) a larger diameter, low RPM pump that will give propulsive coefficients in the range of 0.5-0.6. It will, however, be expensive- which is why we don't often see jets on boats designed for less than 20 knots or so. It is important to note that HamiltonJet and most other jet suppliers will estimate propulsive coefficients for the complete assembly, including all losses within the drive. With props, the figure is more often calculated for the propeller alone- neglecting the drag of shafts, brackets, rudders, lower gear units, etc. To make a fair comparison between jet and prop, the drag caused by the sum total of all running gear has to be factored in with the prop's propulsive coefficient. On this equal basis the two are much more comparable.

As for the hull shape, all the major jet makers ask that the hull be smooth and free of disturbances in front of and near the jet intake. A rule of thumb I've heard from several is to take the length of the intake grate, and ensure that there is no disturbance or irregularity at all for at least that length in front of the intake. Strakes or fins must not line up with the intake. The intake itself can be rotated about the longitudinal axis (ie, it can sit flush with one surface of a 20-degree V bottom) but must stay parallel to the direction of travel. Port/starboard symmetry has to be maintained; thus, if a single jet is used, a flat must be built in to the keel to accomodate it. On a lower speed hull like your 55-65', I'd suggest involving a reputable jet manufacturer right from the start as they can provide a lot of advice on what shapes have been proven to work and what has failed.

[tkinak]

The company I work for runs all jet propelled whale watch vessels. We have four 65' mono hulls with twin 550 hp diesels and Hamilton 362 jets. These are simple light boats with relatively shallow V's and we get 30-35kts wot. With the new engines like the Cat C18 at 1,000 hp you could push your boat pretty fast. If it's wide enough you can mount tripples, there are a couple like that running for New York Water Ways. Weight is the killer, with all the goodies you are talking about you'll need to be careful. Hamilton has a graph and tons of historical data that tells then exactly what you can expect for speed given your vessel weight and hull design. You can find it on their website. They are surprisingly accurate.

Jets would be OK for emergency towing but not on a regular basis. Nothing beats big props and rudders for towing.

Are you thinking electric drives for the jets?

TK

[Toplana]

You can find a lot of guidelines at www.ultradynamics.com

[flydeep]

I have a 50 foot aluminum hull and a set of 303 Ultra Jets that would do a great job for you. This is something you should look at. E mail me at flydeep4u@gmail.com and we could talk off list. My name is Dave
Phone....305-879-0761

[AuxiliaryComms]

Thanks for the offer, but I'm no where close to having a spare dollar to put on anything boat related at the moment.