dual jet drive bottom layout

[Submarine Tom]

On a larger vessel they seem okay but on a smaller hull, quite simply, not

necessary and perhaps problematic in design.

I captained a 20 meter, 54 passenger, aluminum tour boat one summer

that had twin Hamilton 42's driven by twin turbo'ed, twin

supercharged, V16 cylinder Detroit Diesels rated at 1450 hp each. The

performance was impressive in every way right up to 42 knots, but the fuel

consumption was staggering.

baeckmo tells me he's busy with Christmas but may chime in later.

-Tom

[speedboats]

Here is a project I was involved in a while ago, boat is currently operating in the Zambezi river, down stream of the Victoria Falls. Canting the pumps up and onto the deadrise is no problem, but some math is involved it the mainsahfts have a down angle in them to lower the thrust line (also helps the sump clear the bottom of the boat, as CDK pointed out, because having the crank counter-balances swing below the hull is an arse and difficult for oil retension...lol).

The mainshafts must remain parallel to each other, so the intakes are screwed around a few degrees (to compensate for the 5* down bubble in the mainshaft). The amount of angle required does change depending on the deadrise of the vessel at the mount point of the pumps. I kept the pumps as close as the motors would allow, basically a vertical hand space between the headers. There is no specific need to rotate the nozzles to be horizontal, this would in fact create more problems and solves none. One of the larger problems created is the reverse buckets, the nozzle pivot angle will be different to the required operational angle inside the bucket. With a little engineering you could also adapt the bucket, but then it will thrust directly into the transom (rather than under it) instantly removing half of your available reverse thrust (particularly with a ducted bucket)

This vessel had a 10* deadrise at the transom for better lift and to allow the sort manouvers required in adventure-tourisim. From memory the boat weighed in at 2800kgs and had a payload of 22+1pax, and carried 320 liters of fuel (petrol). LOA was 8m and WOA 3.1m. Pumps were Hamilton Jet 212's powered by Mercruiser Vortec 350ci making 330hp each. Vessel was a little under-powered, but a client wants what a client wants. Personally I'd have liked to have seen twin 496's or LS2's. (no such thing as to much power!!!)

As has been pointed out, do not terminate any strakes, place any intakes or imperfections infront of the intake, or inbetween the two. Just think in terms of the intakes needing a constant mouthfull of solid water, introducing air will kill these quick-smart. Straking along the outside edge is acceptable as long as these terminate aft of the intake (preferably all the way to the transom)...


My 2c

[CDK]

Originally Posted by speedboats

There is no specific need to rotate the nozzles to be horizontal, this would in fact create more problems and solves none.

In my post I took a shortcut because of prior knowledge.
I received the jet pumps without any steering hardware and no supplier for Morse cables within a 1000 mile radius. So I decided to move the nozzles with bars, pivoting levers etc. With the steering axis angled, the geometry to maintain identical nozzle angles when turning is very difficult.
I was not satisfied with it once it was ready because steering was heavy, so I scrapped it all, rotated the nozzles and made an internal tie-bar. That made it much simpler and with less friction.

[anthony goodson]

Speedboats, I congratulate you on what must be one of the best posts ever on this site .All practical ,hands on common sense and experience not a theory or equation in sight ,and accompanied by some very relevant pictures, it was a pleasure to read. Just one thing though, please tell me that you fitted handhole extensions before you launched the boat

[yipster]

certainly thanks for the good reply's, very interesting and don't let me stop the conversation but after i suggested asking Alamarin for a lay-out
on asked drawing for a dual jet setup client let me know if Alamarin jet drive will be used it will always be a single middle engine setup
that makes my question in this case academic but did check CSB and PBR boats and noted they again have the jets set more apart..

[speedboats]

Anthony, sorry there were no inspection extensions. This is a trailer boat and it is part of the SOP to not remove the hatches without first being on the trailer.

Did have a clown remove an inspection hatch in a different boat whilst afloat. Was close to the shoreline when he ducked his head down, and had drifted about 50 yards away when he realised he couldn't put it back and raised his head!!! Luckly he managed to paddle closer to shore. We just pulled the plugs, turned her over, did a few flushes of oil, and back in operation the next morn. Unfortunately some operators are more prone to Murphy's Law than others...

[anthony goodson]

Thanks Speedboats ,wish we could get away with that here.At different times of the year we have strings of lobster pots and rows of setnets .not aways well marked and with bits of rope all over them,and then there is the weed and the plastic bags ,et al, with the weed and the bags. the exhaust note tells you first ,quickly followed by the manometer and the temperature guage.Rope doesn't cause an overheat situation ,normally but it has always amazed me how a short length wound around the shaft,completely degrades the jet performance. On this subject I saw an interesting innovation ,new to me anyway, at Seawork this year.I think it was a Swedish jet that had a reverse thread on the shaft,designed to stop monofilament and similar from climbing the shaft and cutting the seals

[speedboats]

Do you guys operate with a grill? Our grills are 5 x 25mm flat bar on it's edge, gaps between the bars is approx 10mm meaning most rope struggles to get in (still happens, though not as frequently), normally blastic bags stay sucked to the underside of the grill (pull bucket down and kill motor normally washes away). Still, nylon line, small sticks and stones are hard to keep out, even a small stick about 8mm in diameter and 100mm long has been known to get stuck in the impellor (particularly the Hamilton Jet 212 turbo option) and cause cavitation. Most of the time we find most of the debris we suffer from will blow through given time, but as mentioned, WATCH THE TEMP!!!.

Yipster, pity they aren't likely to go through with the twin jets project, they are always a fun project and offer all sorts of manouvering options, far superior to any outdrive or shaft and rudder assembly I've encountered.

[anthony goodson]

Yes we do use grills,see photo, the bars on this one are spaced about 2cm and the rear ends of them are not attached to each other ,the theory is that they vibrate like a tuning fork which helps to dislodge debris particularly weed which is a much bigger problem here than stones. If we have a blockage the normal procedure is to reverse as hard as possible ,then kill the engine and lift the bucket whilst the boat is still moving astern this backflushes the jet. I realise this is not possible with engine driven hydraulics ,but it does work with cable operated buckets and those powered by electro mechanical actuators such as mine. This normally sorts the problem ,only once this year did I have to remove the handhole cover whilst afloat ,but even for that once it is worth being able to do that with a single jet. The weed here is mainly in a speed restricted area ,so you don't always notice it straight away,things are much better when you get out to sea.

[Noel Sans]

I have been on hundreds of boats with twin, triple and cuadruple jets mounted over the last 25 years. If the vessel is designed and built correctly you will have a good vessel and excellent manouverbility and with sideways movement for docking. There is no end of advantages.
You will definetly be looking for trouble from the start if you mount jets on a stepped hull. You will find the jets sucking in aereated water.

[Doc Nozzle]

Originally Posted by anthony goodson

Speedboats, I congratulate you on what must be one of the best posts ever on this site .All practical ,hands on common sense and experience not a theory or equation in sight ,and accompanied by some very relevant pictures, it was a pleasure to read.

X2!!! I wish more threads on this forum had posts like that... sigh. But that would be bad because people might actually post more.

[Doc Nozzle]

Originally Posted by baeckmo

There may not be any longitudinal spray strakes or steps inside the buttock line just outside the intake body, nor any transverse steps. Also no through hull fittings like water intakes or outlets, logs or equivalent, in the bottom area in front of the inlet.

And yet there are many race jets equipped with turning fins about 4' directly in front of the inlet mouth.

[Noel Sans]

Originally Posted by Doc Nozzle

And yet there are many race jets equipped with turning fins about 4' directly in front of the inlet mouth.

You will find that you will be getting different max rpm of the engine if you put fins or anything in front of the engine, than if you do not have anything in front. Definetly you will not absorb the correct power and even the possibility to run into overspeed on diesel engines.
It is mentioned earleir that jets should have there shafts paralell on there installation (showing a twin Hamilton jet installation). This is wrong the intakes should be paralell to the keel which also makes the engine closer to the keel.

[jetboaterdave]

Twin jets are nothing new. Fellows have been running them up and down the salmon river for many years now. i've seen more than one twin loaded to the gills with gear and ppl running the river. There are even a couple that have 3 motors and pumps. I dunno about just exactly how to mount them so that they work right but i can tell you if they didn't work very good those guys wouldn't be driving them on some really rough water!!!