Magnum 70 Trying to Achieve 60 Knots
 

Dear All,

What a site to find! Hopefully someone can offer some advice -

I have a project Magnum 70 which in the below configuration is achieving 54 knots. The owner wishes to achieve 60 knots

We have 2 x MTU 16V 2000 M39 upgraded to 2500 RPM coupled to 2 x ZF 3060 (2.03 Gear Ratio) reduction boxes driving 2 x Arneson ASD 15 BiL drives with surface piercing props.
The propellors are Rolla 40" x 54.5" with 5 blades.

Currently the gearboxes output torque is 13804Nm

I have approximately 2 foot clearance from blade tip to bathing platform with the Arnesons in the up position.

The engines are maxed out with all upgrades although it appears the Arnesons can cope with 16948 Nm according to their website.

The yacht currently achieves 56 knots whereas the owner is determined to see 60....

It is possible to retrofit a different ratio ZF box of 2.37:1 which will give a torque input to the Arnesons of 16,116Nm. With this increase would it be possible to fit a larger or more powerful prop set to achieve the desired 60?

Any advice greatfully received.

Matt

Malcolm Phillips at Performance Diesel in Texas would be the man to talk to. He used to set up all the bigger Magnums. http://www.performancediesel.com/ Good luck, Kurt

Many thanks Kurt for your time.It appears there is no email address on that site?

Talk to the diesel experts but the lower gear change you are considering would require approximately 4-5 additional inches of pitch. This would increase torque and throttle response but will most likely not reward you with speed. You may experience a knot or two but I feel 6 knots is a tough one to achieve. Also max rpm levels need to be considered. In other words, if you are quite low on rpm currently, you may have a better shot at the owners goal.

Brett

easy change his GPS from displaying Knots to Displaying MPH and he will see over 60. :)

Depending on how you engines get fuel to the injection pumps you might gain some speed with one of these on each motor.

http://www.fassride.com/shop/fuel-ai...-hd-series.php

I added one of these pumps to my boat and I picked up 2 mph top end and improved acceleration. My gains were due to improved pump timing, and lack of air in the fuel. If the engines are drawing fuel straight from the tanks and don't have any kind of lift pump you will probably see some improvement. For what you are trying to do every little bit helps.

What modifications did you make to the MTU's to get the increased rpms?

All things being relative, your going to need more hp and in the neighborhood of 620 hp more per motor to reach 60 knots. Assumptions: Twin 2,600 hp engines currently in the boat, 45T, your reaching 54 knots, you stated two different speeds above and running the #'s 56 knots is not possible with that setup and 54 knots results in 2.2% slippage so your definitely not over trimmed.

Then your going to need more pitch in the propellers, more power and a lot of money to get this extra speed.

Or.... You can do what everyone on South Florida does to increase speed. Run the boat and head for a shoal of with about 2-3M of depth and watch your speed increase 2-3 knots. Remove the bottom paint and gain 3-8 knots+, run it nearly empty and without gear, people, supplies, etc. and on a cold day with a fuel cooler on the engines.

I understood the rest can you explain this part just curious, that is basically all of Biscayne Bay. I am guessing this is only for large boats and something to do with pressure waves under the boat.

Or.... You can do what everyone on South Florida does to increase speed. Run the boat and head for a shoal of with about 2-3M of depth and watch your speed increase 2-3 knots. Remove the bottom paint and gain 3-8 knots+, run it nearly empty and without gear, people, supplies, etc. and on a cold day with a fuel cooler on the engines.[/QUOTE]

Brother, say it like it is................................................ ..LOL

Any item that is placed into the water displaces its volume to weight.

When any boat is in deeper water the boundary layer of the buoyancy is displaced outward into the water. When you run these, especially larger heavy boats, in the shallow water the boundary layer of the buoyancy is reflected off the sea bed thus pushing the boat back upward (somewhat) and reduces the amount of the boat that is in the water. We are not talking hydrofoil here but neither are the gains. It's basically a "ground effect" for the marine environment.

Note: these boats have a 1M or so draft as this boat in question has a 45T displacement and the boundary layer is quite large.

The same theory applies to a sky diver or vehicle. The object going through the air is pushing against the air ahead of it and this stacking of the air ahead of the person or car slows down the person/diver to a point that the diver will reach a terminal velocity of around 140 mph. An automobile has an engine that can produce more power so it can overcome the air stacking to a point whereby the drag = the thrust and no more speed is obtainable with the given hp. Or just visualize the ground effect that a Sea Gull uses to float off the water surface with very little required wing flap.

Being that the density of water is some 784 times greater than that of air you can see whereby running 45T into a shallower marine environment can form a boundary layer of buoyancy and thus create lift on the displaced vessel.

This is a very common trick the boat salesman do in South Florida. Or at least the smarter ones!

As for lighter boats the boundary layer is not nearly as large so you would have to go to some really shallow water to be able to achieve any benefit and the odds of contact greatly increase.