To answer the OP-

Horse Power = the ability to move stuff.

There’s voodoo math involved when boats come into play.

All that HP on the back of the boat “should” move that boat easily.

Here’s the weird part. “Should”…
Now you’re dealing with propellers. Propeller size and pitch have a lot to do with the boat and its speed or torque. I can’t recall off the top of my head, but ***I Think*** the higher the ratio the faster the boat, the lower the ratio, the more torque.
Kind of like figuring out gear ratios in transmissions and rear ends when it comes to cars and trucks.


Here is a helpful video to summarize:

https://youtube.com/shorts/Rsq...?si=cAPy5fDE-mI1TelX

I run a lot of these boats mentioned.

Diesels- most don't want diesels in these large center consoles because they draw more water (can't get into shallower places. Also the diesels are usually shoe-horned into the center console and very hard to work on and with outboards a cabin and head are in that space. Outboards are much quieter, faster, and no diesel smell. Overall diesels are more reliable and fuel efficient in general.

A large center console needs a certain amount of motors just to have enough propeller blade area to get on plane and stay on plane. When you lose a motor (with a lot of motors) you can still get on plane and run fast, lose 1 of 2 diesels and you're limping home at 10 mph. People like to go fast and get there quickly, fishing tournaments it matters a lot. A lot of these fishing tournaments pay out 6-7 figures for 1st place, and cutting down on running time by being fast may make the difference from winning 1st place and winning nothing.

I can go from Boat Harbor, Bahamas 200 miles to Florida in 4 hours easily in one of the 39' Invincibles I run back and forth for the owner. Nassau to Fort Laud in 3.5 hours (185 miles) in a 34' Nortech. Can do the same trip in a 37' Midnight express with 4-500R's in 2 hours or so if it's calm enough......or 6 hours in a 70' Sportfish I run.

But on the Sportfish you're taking the entire house out fishing, so if women or non fisherman on the boat, they're getting off at 7am for the day or getting bounced around. The center console guys rent a house on land, and can also sand bar hop, island hop, fish, snorkel etc, where some of that isn't possible with the sportfish.

At a certain point even to me a center console over 42' doesn't make any sense. I brought a brand new 2021 Hatteras 59' GT Sportfish with 3 staterooms (bedrooms) and 3 heads (bathrooms), galley (kitchen) with 2 generators (1 runs the entire yacht) and 2- 12 cylinder 1600HP Caterpillars it came in at $3.2 million. Pulled into Fort Lauderdale and docked it next to a new 52' Scout center console with 5 outboards that is 1/3 the boat and that sold for $3.5 million.

Very few outboards offer different gear ratios in the same HP. You use different Pitch propellers depending on top speed to get the rpms needed for the engine's powerband depending on the boat they're on.

I am repowering my 96mph boat, it has a supercharged 6 cylinder Mercury 400R (HP) rev limiter is 7050rpms with a 1.75 ratio, it does 96 at 6850rpms with a 29" pitch prop. I'm putting a new 2026 400R which is a v10 naturally aspirated with a 6650rpm rev limiter and 1.60 ratio. I hope to pick up 4-5 mph, so should be 6550-6600 rpms with same prop and 100 mph. The V10 has a lot more torque so doesn't need the lower 1.75:1 gear ratio or come with it.

Generally on a 4 engine boat (this is if it performs fairly well with 2 engines) the 3rd engine gives you 12% more speed and the 4th engine gives you 8% more. Adding motors isn't as cut and dry as simply adding up the HP, because each additional motor has the water drag of another lower unit and propeller in the water. IF weight were the same, 3-400's would be 10-20% faster than 4-300's usually on the same boat (and in this case the motor weight equals about the same).

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quote:

Originally posted by CPD SIG:

There’s voodoo math involved when boats come into play.

For sure with a cube root relationship, plus the weight of the boat being part of the equation for speed vs hp!

In aerodynamics it is a squared function (or square root if you re-arrange the equations). Weight is buried into the coefficients but is a minimal contributor.

One of the things which I find fascinating, is that off-shore racing sailboats have radically changed in design, and have become much, much faster over the last 15 years or so.

Demographic collapse etc may well be part of the reason why yacht prices are stagnant/the wild jump during COVID - but part of me looks at how differently racing hulls are, and how much faster, and thinks that in 10 years or so, cruisers will be almost twice as fast.

They are starting to look almost cetacean - at least the monohulls/finally look like something which couldn't be made from wooden planks.

I wonder if motorboats will have a similar change... C foils and a scow bow would make for an interesting trawler...

Aglifter

There was kind of a... "war of escalation" with the Narcos and Law Enforcement when it came to hull designs a few years back. Donzi was involved.


https://en.wikipedia.org/wiki/Donald_Aronow

For the TL/DR crowd: He dead.

Don't some aircraft offer variable pitch props ,?

quote:

Originally posted by bendable:
Don't some aircraft offer variable pitch props ,?

Yes. Actually they are constant speed from the pilot's perspective, where you set the desired rpm and the governor mechanism adjusts the blade pitch to maintain that rpm.

I've wondered if some boat props have variable pitch.

quote:

Originally posted by Fly-Sig:

quote:

Originally posted by bendable:
Don't some aircraft offer variable pitch props ,?

Yes. Actually they are constant speed from the pilot's perspective, where you set the desired rpm and the governor mechanism adjusts the blade pitch to maintain that rpm.

I've wondered if some boat props have variable pitch.

They tried variable pitch propellers and it didn't work out too well in the recreational market. Never seemed to take off. The Mercury v12 600HP outboard does have a 2 speed gearcase, and dual counter rotating propellers. A few Suzuki outboards also have dual props.

A few expedition type yachts have variable pitch propellers, but very rare as maneuvering with them is the problem, cannot go from forward to reverse quickly.

Some commercial ships utilize them.

quote:

Originally posted by jimmy123x:
They tried variable pitch propellers and it didn't work out too well in the recreational market.
...
Some commercial ships utilize them.

Thanks!

quote:

Originally posted by Fly-Sig:

quote:

Originally posted by bendable:

Don't some aircraft offer variable pitch props ,?

Yes. Actually they are constant speed from the pilot's perspective, where you set the desired rpm and the governor mechanism adjusts the blade pitch to maintain that rpm.

There are some variable pitch props that are not constant speed. Back in the 1960s I instructed in a client's Stinson that had a variable pitch prop that was not equipped with a governor. The prop speed would change with each throttle change, so power adjustments involved manipulation of two controls, throttle and prop.

ETA: Now that I think about it, early Bonanzas had variable pitch, non-governed (not constant speed) props. Blade pitch of modern props is via hydraulic pressure; the early Bonanzas did not have hydraulic pitch control; the pitch was controlled by electric motor.

quote:

Originally posted by V-Tail:
There are some variable pitch props that are not constant speed. Back in the 1960s I instructed in a client's Stinson that had a variable pitch prop that was not equipped with a governor. The prop speed would change with each throttle change, so power adjustments involved manipulation of two controls, throttle and prop.

ETA: Now that I think about it, early Bonanzas had variable pitch, non-governed (not constant speed) props. Blade pitch of modern props is via hydraulic pressure; the early Bonanzas did not have hydraulic pitch control; the pitch was controlled by electric motor.

I'd heard rumors of variable pitch non-governed props but never flew one.

I'd have to say you ain't all that great at spelling, either!

What other considerations are there?

Weight of additional motors.
Weight of additional fuel.
Is there additional drag ,because the boat sets a wee bit lower?

Do half of the motors turn counter clockwise to offset the other half?

quote:

Originally posted by bendable:
What other considerations are there?

Weight of additional motors.
Weight of additional fuel.
Is there additional drag ,because the boat sets a wee bit lower?

Do half of the motors turn counter clockwise to offset the other half?

Other considerations- There are a ton actually. Usage would be the primary, are you just buying a big center console to go fast and get places. Or is it primarily a fishing boat, but still want speed. The other big consideration is where. In the U.S. or are you spending a LOT of time in the Bahamas or Mexico where fuel quality is also a determining factor. Warranty- right now Mercury Marine is offering 3 years warranty, plus additional 4 years of gold (most items except electrical), Racing is only offering the 3 years.

The go fast guys will generally run a much lighter boat. The fishing guys will load the same boat up with 2000-4000 additional pounds to fish a tournament, 8+ guys, tons of ice, several hundred gallons of water in the livewells, 20 rods/reels, tackle, etc. etc.

Mercury racing is a totally different division than Mercury Marine. The new higher HP motors always come out from Mercury Racing, that HP (in a certain size/weight engine) might only migrate over to Mercury Marine 1-2 years later. The engines are denoted with a R after the HP. The other issue is you have a choice on different Lower Units with Mercury Racing One motor you may be able to pick between 3 or 4 different lower units better suited to your application. Or engine weight. Some Mercury racing engines are rated for 87 octane and others require 89 or 91 octane (which one may not be able to get in their area).

The 300R and 400R are Naturally aspirated and rated for 87 octane. However the 500R is supercharged and based off of the 300R and rated for 89 octane. Another is lower units, if you're going much over 80 mph, you need a lower unit designed for that, hence choosing a Mercury racing engine with appropriate lower unit. If you don't, you could have a blow out, meaning engines lose traction and the boat violently turns very sharply usually resulting in throwing people overboard and damaging the boat. The lower unit on my boat is a sportmaster, and once on plane only the bottom half of the propeller is in the water, and has low pickups for the cooling water of the engine and a hydrodynamic shape.

Another is weight, Mercury Marine makes a v12 600hp, has 2 counter rotating props and a 2 speed gear case, but weighs 1276 lbs!!!!! A 500R weighs 726 lbs, so depending on your usage, you may want an extra 500R over less 600's. Mercury Marine makes a v10 425hp at about the same weight as a 500R and then skips to 600HP at a HUGE weight penalty. So someone may want 1 more 500r over 600's and weigh less and be faster.

WEIGHT of additional fuel- generally almost all boats in this category have the same fuel capacity so that's not an issue, but motor weight is. The extra motor a lot of times pushes center of gravity aft, and the outside motors are mounted really close to the hull side, so you can't fully use the trim tabs to add stern lift because then the tabs deflect water away from the outside motors and they start overheating. So the boat with 1 less motor will generally ride better when it's rough.

No extra drag from boat sitting lower. But each additional lower unit and propeller in the water adds the extra drag and adding motors starts becoming diminishing returns.

Yes, you have normal rotation and counter rotating lower units, so half of the propellers turn right hand and half will turn left hand. I skipped over some stuff but it's the gist of it.

I ran a 37' Midnight express with 4-500R's, stern sat pretty low when full of fuel. Took a long time to get on plane with bow up blocking visibility for about 10-15 seconds. Had only 2' with occasional 3' on starboard bow for a run from Miami up to Palm Beach 55nm or about 63 statute miles. Started doing 35 mph ride was good, bumped it up to 45 mph ride still good turning a little over 3000 rpms (WOT 6600 and normal cruise for these engines is 4000-4500 rpms), bumped it up to 55 mph and boat still ran good but barking the props on the 3'ers (props losing and gaining grip on outside motors, can be hard on lower units), owner asked me to go faster bumped it up to 65 mph, launched the entire boat 5' in the air for about 50-75' a few times, brought it back down to 45 mph. It's not often you can really wring one of these super fast boats out in the ocean, another consideration. But 4000 rpms is 60 mph (which is an easy cruise on the motors) and fuel mileage is good getting about 1 mpg or better. Meanwhile the owner isn't really experienced to drive it, the company gave him some lessons but driving a boat like this takes years of experience to really get it.

This is what happens on a big/fast center console if you have a blow out (lower units lose traction) or you hook a step. Things turn ugly and SHTF instantly, this boat went over 90 mph.

https://www.miamiherald.com/ne...rticle279828094.html

Here's the fastest center console, I know the guy. 128 MPH, he has quad 400R's with nitrous. You can see how it starts to want to get sketchy when he's slowing down.

https://www.youtube.com/watch?v=mlhxzQ7UQIU&t=1s

I have another client that is approaching 80 years old and his 50' boat does 224mph in a mile and he drives it himself, with a throttle man.

This is my buddies boat, same boat as mine but has a tuned 450R, it's done over 124mph, I've ridden on it. I just have a 400R on mine.
https://www.youtube.com/watch?v=Q3nEnUDQNxk

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