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Is the Multi-Outboard Trend Justified? - Part One

By: Captain Bill Jennings

Nor-Tech 340 Sport with triple 400R's
Nor-Tech 340 Sport with triple 400R's

An in depth look at the current trend to add more outboards to boats – Part One

Less than a generation ago most boaters were happy with a single outboard on their transom, but as the demand for horsepower grew they turned to ‘sterndrive’ systems that offered more power by using auto engines. But the horsepower in outboards has steadily grown and outboards are back in vogue. Recently, a new trend is appearing in the world of boating -- multiple outboards on the transom.

Want power? Just clamp on an extra three or four outboard motors. Depending on where you look, the trend is going wild. This begs the question: Is adding more engines a practical and logical source of performance, or is it just a costly horsepower solution for wealthy boaters who want to look good?

Last year I asked a boat owner with four 300’s on his transom why he did it. His answer was simple; “Because I can.” So lets take an in depth look at why boaters are lining their transoms with three, four, and even nine outboards and determine if such a layout is actually smart. Because the subject of multiple outboards is very complex and not a simple multiplication of added horsepower, I will limit my discussion to a maximum of four outboards on a boat. A good place to start the discussion is listing the general pros and cons of adding motors to your boat. If you compare these multi-engine advantages against the type of boating you do and where -- you can calculate if having more than one outboard on your boat is a good idea.

If you boat in sloppy water, you will find that more than one outboard is an asset. The application of thrust is more consistent because you have additional props operating efficiently with reduced ventilation and slippage. Of course the added power will also provide greater flexibility to travel faster to your destination and return home faster should the need arise. At the same time, having one or more 'back-up' motors on your boat can provide the peace of mind necessary to venture further offshore with less concern about being stranded should your single engine fail.

Nowadays, outboard motors are more reliable and require less maintenance than ever before. Nonetheless, no matter how small the likelihood of an engine problem with one engine, you can multiply that possibility by the number of outboards on the transom. This added maintenance cost must be factored into the cost of going with multi-engine setups. Some people argue that engines on a multi-engine craft have less overall engine stress because the workload is spread. Arguably, this will result in each engine being more reliable and lasting longer. I have not seen any quantitative research on this point.

One known fact about multiple engines is that each added engine means more potential mechanical problems. And it doesn’t end there. With the added complexity of multiple engine installations you will need to find a qualified dealer to perform your maintenance, and the restricted work space makes simple jobs more difficult and therefore more expensive. Dealers may find this potential for greater repair revenues a bonus.

If you are adverse to noise while boating, remember that more engines equals more noise. Yes, today’s outboards are relatively quiet, but you still increase sound levels with each engine you add.

Greedy insurance companies love multi-engine boats. Their one sided algorithms say that the risk of insurance claims is far higher for multiple engine boats. Even if statistics could prove to actuaries that triples are as safe as singles, they still know that the guy with triples can afford to pay more for insurance.

Some of those initial costs may be recovered when you come to sell your dual or triple powered boat. It seems boats with more than one outboard are more sought after by used boat buyers. Buyers that would like a multi engine boat, but balk at paying the price of a new one, look to the sharp depreciation on boats to justify buying a two or three year old model. The pandemic has pumped the boat market over the past year, and some of these owners will soon discover that running a flock of outboards is more costly than they thought. If the boater tries to rectify this cost concern by boating less, they are defeating the purpose of owning a boat.

What about the added load on the boat? Boaters look at a center console boat with four heavy outboards and ask, “How can the transom handle all that weight?” A large OB such as a 300 hp will weigh in at around 620 lbs, which means that quads put 2,480 lbs of load on the transom. But manufacturers are well aware of such strength requirements when they are designing the boat, so we have not heard of any scenarios where a transom split and dropped it’s load. No worries here!

The extra weight difference to move from a triple to a quad, does however add considerable weight to the back of the boat and this changes the boat’s ‘center of buoyancy.' If you wanted to set up your boat for optimum balance and center of gravity with multi engines, it may be a requirement to make modifications to the planing surface of your boat’s hull. Since manufacturers position their boat hulls as “one size fits all," it is reasonable to assume that a specific boat will work best with one of; twins, triples or quads, and not as well with the other two. Tabs can help, but this amounts to a poor fix and improperly adjusted tabs that will rob performance.

Most boaters are interested to learn how their boat would perform with an added engine. Searching through multiple performance reports it is no surprise that to obtain accurate numbers you must perform such tests on specific makes and models, and the results will vary. Some tests report that adding a second engine does not increase speed because of the added drag from the second engine’s lower unit. However, most tests that I have read show that speed increases with a second engine. Rob Watson, owner of Watson Engineering, in Ridgetown ON, showed me the following test on a Scarab 34 with which he experimented:

Rob figures that the added transom weight increased the angle of attack, providing additional lift, which in turn reduced the wetted surface along with drag. But the four engines at full throttle drank 112.5 gallons per hour and with only a 100 gallon fuel tank, it made for a short day on the water.

Terry Sobo, of Nor-Tech High Performance Boats in Fort Myers FL offered some interesting comments about the trend of triples and quads. He said that a lot of his center console buyers have a high performance background and so that while they want a larger boat more suited to offshore applications, they still want the ability to go fast. He thought that with today’s larger engines twin motors would suffice, but for what amounts to a relatively cheap $35k more they gravitated towards triples. NorTech being a popular builder of multi engine outboard boats, gave this performance chart for their 34 foot performance center console, using 350 hp outboards.

I should note that because NorTech specialize in fast and efficient hull designs, not all boat manufacturer’s performance numbers will be this strong. As a general rule for all boats -- each added outboard will add between 6 and 10 mph to the overall speed.

Another plus for triples and quads is the performance you can expect if and when an engine falters. If you have twins on a boat over 30 feet and lose an engine, you won’t get back onto plane with the remaining outboard. With triples you will. I’ve spent many hours idling back to port on one engine after one of my two engines failed in a poker run or a race.

If you have been keeping track of the pros and cons of multi engines to this point, you may be thinking that triple and quad outboards have their place but a single outboard is still the norm -- making multiples and 'more toys for wealthy boys' the ultimate answer. In part two of this review, we will look at some technical arguments for and against adding outboards to your transom, and that will make the answer more clear. Stay tuned.

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