Hamilton waterjet/ Volvo Penta D6 / 36' hull / Speed 40 kts?

[nordic]

I am initiating a new project that plan to use the above components - the boat design parameters at this stage are based around the fact that the boat top speed should be around 39-42 knots. Parameters at present points at: Glassfiber hull length 36-37' , weight of hull 2500 lbs, weight of finished boat all inclusive about 7000 lbs, Volvo Penta D6 435 HP Inboard to drive the suitable model of waterjet from Hamilton.
I do realise that in order to calculate within a few knots the estimated topspeed, more precise data is required so at this stage I just want to ask anybody with knowledge or experience if a boat with roughly the above parameters would be able to reach a topspeed of 40 knots. It is an fairly open workboat - currently we have a 31' hull weighing the same as the above and driven by the Volvo Penta D6 370 HP with a sternleg, this boat does 40 kts topspeed but I am new to waterjets and needs a bit of advice on whether the above parameters sounds about right for the project or I need to rethink? If you as a bonus also have any ideas of how the boat would behave acceleration wise compared to a boat of similar parameters driven by a sternleg, that would also be interesting to hear about. Any advice appreciated!

[Ad Hoc]

you have a hp/ton of around 135, this should just about get you to 39~40knots, if you're lucky and the hull is designed for that speed and your weight does not increase above 7000lbs. Just your propulsion train, eng + jet is around 2~2500lbs, add that to your hull you ahve around 5000lbs already, not much margin for fuel, outfit etc not errors and weight growth.

But without drawing up a sketch and doing a half decent weight estimate,of everything on-board, it is all just pure guess work of its weight and final performance.

[Lt. Holden]

Did I miss something or did you say a fiberglass hull weight of 2500 lbs for a 36 - 37' foot boat; how are you going to do that? Inquiring minds would like to know.

[nordic]

The estimated weight of the hull was for the shell only and based on a 35 foot hull that is produced in fiberglass and Divinycell foam in a sandwich construction. ( weight information supplied by the manufacturer ) I forgot to mention that that was an estimated weight of the shell only not including reinforcements and deck, consols etc. There is no top or cabins on this boat so hope is to keep the weight of the hull, exclusive fittings, engine, jet etc. to around 3800 lbs or 1700 kgs, does that sound more right?
The updated calculations and half decent weight estimate now shows an overall weight of about 8.000 - 8.500 pounds including some fuel and one person onboard - it is not expected that the boat has to do 40 kts with full fuel tanks and more people onboard.
Also, the length of the hull will be between 35,5 - 36 feet, so 4-5 feet longer than a boat of very similar design we have today - the design parameters that will change is: Add 4-5 feet to length, increase engine size from 370 HP to 435 HP and change propulsion from Sternleg drive to Waterjet. The total weight increase between the two boats will therefore be + 200 lbs for the waterjet unit and drive shaft compared to the weight of the sternleg and transom plate, as well as the additional weight of the added 4-5 feet extended hull. This could be adding maybe 800-1000 lbs? meaning that the overall weight would be increasing with 1200 lbs compared to our boat today. The Volvo Penta D6 is a very powerful engine and we can load our existing boat up quite a bit without seeing a big drop in top speed. Does this additional information provided help anybody to give me their five cents worth of opinion on the outcome?

[Ad Hoc]

nordic

"...does that sound more right?.."

Well, if you don't know, why are you asking us, when we have even less details about the boat than you?

But based upon your now changing SOR you're hp/ton is less at about 110 and as such you're going to get a max of around 35~36knots.....as I said previously, weight kills. Which Lt. H also mentally prodded you to ask yourself or at least justify too..

[Lt. Holden]

Perhaps some professional designers can weigh in here, you are talking about an estimated gross weight (4x or more) than the weight of the bare hull; is that Kosher or advisable?

[Ad Hoc]

Lt.H
With out any specs or dwgs, it is impossible to ascertain the weight of the boat, period. Hence if he cannot decide what the weight will be, why does he expect anyone else, in the absence of any hard data.

[RivrLivn]

I have a 21' boat with a hull weight about 2800 lbs dry.
I have a 310 hp V8 with Hamilton hj212 pump.
Full throtle 4200 rpm I can't reach 42knots, maybe 40knots going down steam.

The Hamilton pumps are not "speed" pumps like American Turbine's.

They pump a high volume of water and have great manuverability for a jet.

I just can't imagine that size boat with that hp doing 40+ knots.

Remember I'm no expert only a boat owner.

[nordic]

I appreciate the feedback and read inbetween the lines that this is doubtfull. The question here can be changed a little in order to hear what someone with water jet experience thinks: A known boat of 31,5' with a Volvo Penta D6 370 HP and a sternleg does 40 kts+ topspeed. Now take the same hull shape, modify it by extending the waterline 4-5 feet, increase weight by 1200 lbs, increase HP by 65. I am fairly confident that this new theoretical hull if powered by the Volvo Penta D6/ 435 HP with a Sternleg would still do 40 kts. Topspeed of our 31,5' hull loaded with 1200 lbs extra does not drop more than a few knots, you should think the increased waterline of the 35-36' hull would improve performance? Does the performance of a waterjet propulsion system "steal" such a significant amount from topspeed that I should expect only 35-36 knots?
I will off course seek proper advice once the design parameters seems they could meet the requirements, question then would be how to achieve this, could shorten boat 1-2 feet and possibly reduce weight of fittings by 500 lbs but as of now I am fishing for free and unbiased advice and input, before committing with the boatbuilder, from someone that has built boats around this size and equipped them with a waterjet. I do realise that there are many "if's" and "floating facts" in my post - that is the nature of initiating a new project where parameters are just being considered - try to be constructive rather than critical in any feedback and I will maybe just find a way to make this work

[speedboats]

Trying to compare mixed flow pumps (for pressure and velocity) with axial flow pumps (high mass/ torque) here is futile with the size of the mooted project. Using the Penta D6 (nice compact power plant) would mean going to something substantially bigger than the HJ212, and therefore the AT312 (American Turbine class equivilent) would be an out-to-lunch suggestion. You'd most likely end up with something like an HJ271 or HJ292. Both have a max RPM of around 2-3000rpm and built for the diesel setup. The HJ292 is rated to 500hp (diesel) depending on impellor option, and claim to push 10,000lbs better than 25knts, but your best bet is to ring the guys at Ham jet in Christchurch, NZ, and have this conversation with them, it's what they do all day, every day.

[Ad Hoc]

For adding 1200 lbs with just an increase in 65 hp, the hp/ton is not the same as the original boat, it is less, hence your total hp/ton is reducing, ergo your speed wont be as fast.

Speedboats, i assumed the 322's, since he needs a large as possible jet he can install. Which is why i questioned the weight of the outfitting, that's before even addressing the hull, and its SOR for its own weight!

He doesn't need to talk to boat builders...he needs to design the boat, obtain some hard facts. Too many unknowns and too many "what if's".

[speedboats]

I wasn't meaning to talk to the boatbuilding side of Hamilton Jet (which is dedicated to small run-abouts), but rather its waterjet manufacturing division. Just thinking that they may know something about their own pumps... It is also most likely they will/ have asked the same questions as above asked by yourself, and without a design is really a 'how-long-is-a-peice-of-string' question

[CTMD]

If you contact hamilton they will ask for:

Engine power and RPM, Boat weight, Hull type, deadrise (assuming typical monohull). Based upon this they'll give you a ballpark jet sizing and speed. However you need to trust much of the advice given above as its right and will save you time. Adding 5 feet to a 31 foot boat could increase your hull weight by up to 25% then you've got extra engine and jet weight as well. Boats don't scale linearly so adding 15% to the length will require much more than 20% more power for equivalent speed.

[Russclif]

I have a 32 ft deep V boat, weighs in at 5,300kg, powered by 460hp, direct drive through a 191 Hamilton Jet Unit. Performance as follows: at 70% loading - 25 knots, WOT = 32 knots. Sea conditions smooth no wind.
For your sort of performance you need over 100hp/tonne.