Bertram 31 Flybridge Cruiser

Built 1961–1983 by Bertram Yacht

Own one? Keep her maintenance, manuals and checklists here — free for a week Shopping? Find the right used boat Looking at one? Sea-trial scorecard

About the Bertram 31 Flybridge Cruiser

Researched from public sources for this model. Any individual boat may have been modified or repowered — confirm before relying on any figure.

Illustration of a Bertram 31 Flybridge Cruiser — **Illustration, not a photo of this model.** AI-generated illustration of a typical convertible sportfisherman.

Built: 1961–1983
Builder: Bertram Yacht
Length: 30.58 ft
Beam: 11.17 ft
Draft: 3.08 ft
Displacement: 10,600 lb
Hull / keel: planing
Propulsion: inboard
Fuel: varies by year
Water: 18 gal
Holding: 12 gal
Generator: often added later
Bow thruster: not fitted
Stern thruster: not fitted
Steering: cable

Tank capacities — where the figures come from

Fuel varies by year
“Fiberglass fuel tank capacity 170 gal.” — brochure (1969 (brochure printed 1/69)), bertram31.com · “molded fiberglass tank with a capacity of 170 U.S. gallons” — owner's manual (early boats (manual drawings dated about), bertram.com · “In 1968 fuel was increased to 222 gal” — owners' association (later boats (1968 or 1972 on)), bertram31.com · “175 gal until 1968” — owners' association (early boats), bertram31.com · “231 GALS. FUEL CAPACITY” — owner's manual (one hull (312-16-1235), gas MerCruiser), bertram31.com

Bertram's early owner's manual (on the builder's site) and the 1969 brochure both give 170 gal, so the owners' site's 1968 date for the increase is wrong and its other date, 1972, fits. For later boats the owners' site says 222 gal, but the factory data sheet in one hull's later manual says 231. Our earlier 222 rests on the owners' site alone.
Water 18 gal
“Fiberglass fresh water tank capacity 18 gal.” — brochure (1969), bertram31.com · “18 Gals. WATER CAPACITY” — owner's manual (one hull (312-16-1235)), bertram31.com
Holding 12 gal
The 1969 brochure lists a manual marine toilet only; no holding-tank figure was found.
Water heater unknown
“FRESH WATER SYSTEM: The fresh water tank ... has an 18 gallon capacity ... GALLEY WATER PUMP: Manually operated at sink.” — owner's manual, bertram.com

The builder's Bertram 31 owner's manual describes the whole water system as an 18 gal tank and a manual galley pump, with no pressure pump or water heater; shore power and a converter were options. That points to no factory water heater, though the manual doesn't say so outright.

Tanks are often replaced or added on older boats — check the tank labels or a survey for yours.

Thrusters and steering — where this comes from

Bow thruster not fitted
Our earlier figure of none has no source. The builder brochure lists standard equipment (no thruster) but refers to a factory price list for options, and the owner's manual doesn't mention a thruster.
Stern thruster not fitted
Our earlier figure of none has no source, and no option list was found.
Steering cable
“Steering is controlled by a steering wheel which is directly connected to the rudders by a push-pull type cable which ru” — owner's manual, bertram.com

Bertram's owner's manual (from the builder's legacy library, same text as the owners' site copy) describes push-pull cable steering. The manual is undated and covers the whole 31 range, so later production years (to 1983) aren't separately confirmed. The cable maker isn't named.

Thrusters and steering are often replaced — check the units on your boat before ordering parts.

The original production deep-V; over 2,000 31s of all versions built (hulls_built covers all versions, not just Flybridge Cruisers). Regular production ended 1983, plus a 1986 Silver Anniversary run (HMY says 23, BoatUS 20). Fuel 170 or 222 gal. The 31 Sport Fisherman is the same boat without the cabin bulkhead. Build count (approximate, not stored): HMY: over two thousand 31s of all versions built.

Engines fitted

MerCruiser gas V8 inboard (model not stated) — 330 hp, gas, twin standard
HMY: twin 330 hp MerCruiser gas engines powered the majority of 31s (22-24 kn cruise, 30+ top); the owners' site spec page lists the same.

Sources: hmy.com · bertram31.com
Detroit Diesel GM diesel (model not stated) — diesel, twin factory option
HMY and the owners' spec page say several GM, Cat or Cummins diesels were offered; no model or rating given. Owners discuss 4-53/6V53 Detroits mainly as later retrofits.

Sources: hmy.com · bertram31.com
Caterpillar model not stated — diesel, twin factory option
Listed by HMY among the optional diesels; no model or rating found.

Sources: hmy.com · bertram31.com
Cummins model not stated — diesel, twin factory option
Listed by HMY among the optional diesels; no model or rating found.

Sources: hmy.com · bertram31.com
Cummins 6B-series (210-270 hp) — 270 hp, diesel, twin common repower
On the Bertram 31 owners' forum the 270 hp Cummins is the most-mentioned diesel repower; 210-220 hp versions are suggested for keeping original shafts and exhausts.

Sources: bertram31.com
Yanmar model not stated (about 240-250 hp) — 250 hp, diesel, twin common repower
Yanmar diesels appear on the owners' spec page and in the repower thread (250 hp mentioned).

Sources: bertram31.com · bertram31.com

Boats of the same model can differ — check which engines yours actually has before ordering parts.

Generator

often added later

“common modifications include: addition of a generator, transom livewell / fishbox, diesel conversion, air conditioning s” — owners' association, bertram31.com

The builder's owner's manual has no generator section (12V system with optional shore power and converter only). The Bertram 31 owners' site lists adding a generator as a common owner modification; no make or size is typical enough to list.

Manuals and brochures online

Owner's manual Bertram 31 owner's manual (early edition, 67 pp. scan) - Bertram legacy owners manuals — bertram.com
Brochure Bertram 31 range brochure (Sport Fisherman, Flybridge Cruiser, Bahia Mar, Express, Hardtop), printed 1/69; labelled 1972 on the owners' site (1969) — bertram31.com
Owner's manual Original Bertram 31 owner's manual (64 pp., with factory data sheet) — bertram31.com

Hosted by the builder or an owners' association; they own the copyright.

Known problems for this model

Fuel tank replacement fuel
Many 31s have had original tanks replaced; owners stress that aluminum replacements fail early from trapped bilge water, abrasion and rubber-on-aluminum contact unless coated, properly supported and kept clear of standing water.
bertram31.com
Wet ride hull
The classic deep-V is an exceptional rough-water boat but a wet ride.
hmy.com

Engines & parts

MerCruiser 5.0 (305) / 5.7 (350) V8 gas parts: good

Factory standard engine — you entered “MerCruiser gas V8 inboard (model not stated)”

GM small-block Chevrolet base engines, so internal parts are everywhere. Marine exhaust manifolds and risers for 1983–2002 small blocks are sold by many aftermarket makers (Sierra, Barr, Osco and others) through Power Boat Supply, Hardin Marine, Basic Power Industries and similar stores; Mercury sells genuine parts through dealers.

Parts sources: Power Boat Supply, Hardin Marine, Basic Power Industries

Common problems (2)

Manifold/riser failure and water in cylinders exhaust
Cast-iron manifolds and risers rust internally or their gaskets fail, letting water back into the cylinders; small amounts cause hard starting and rough running, large amounts hydrolock and wreck the engine. Service roughly every 7 years in fresh water and about 5 in salt.
powerandmotoryacht.com · forums.iboats.com · justanswer.com
Overheating from scaled manifolds cooling
Rust and scale inside the manifolds restrict cooling flow and cause overheating under load.
powerandmotoryacht.com

Maker’s service schedule (8)

Engine oil & filter change	100 hrs or 1 yr	“Most four-stroke outboards and sterndrive engines require an oil change either annually or every 100 hours of operation.” — manufacturer, mercurymarine.com
Fuel filters & water separator	100 hrs or 1 yr	“The fuel filter on your engine should be inspected or replaced per the maintenance schedule in your owner's manual, usua” — manufacturer, mercurymarine.com
Raw-water pump impeller	300 hrs or 3 yrs	“How often should my seawater pump impeller be serviced? Every 300 hours or 3 years, whichever comes first.” — manufacturer, mercurymarine.com
Spark plugs inspect / replace	300 hrs or 3 yrs	“How often should spark plugs be changed? Every 300 hours or 3 years, whichever comes first.” — manufacturer, mercurymarine.com
Coolant replacement & freeze-point test closed (freshwater) cooling only	5 yrs	“How often should coolant be changed on engines with closed cooling? Every 5 years.” — manufacturer, mercurymarine.com
Transmission fluid change sterndrive gear lube	1 yr	“Gearcase lubricant in outboard and sterndrive lower units should also be changed annually or at the service interval spe” — manufacturer, mercurymarine.com
Lubricate steering linkage and check power-steering fluid sterndrive	100 hrs or 1 yr	“Mercury Marine recommends lubricating the steering linkage and checking the power steering fluid level in ... MerCruiser” — manufacturer, mercurymarine.com
Check power trim fluid level sterndrive	100 hrs or 1 yr	“Power trim and tilt fluid level should be checked annually or every 100 hours.” — manufacturer, mercurymarine.com

Service part numbers (6)

Oil filter	Mercury Quicksilver 35-866340Q03	“Part number: 866340Q03 ... for MerCruiser sterndrive and inboard engines except V6 engines with the oil filter mounted o” — manufacturer, mercurymarine.com · “Includes: Oil Filter Oil Filter 866340Q03 Fuel Filter 802893Q01 ...” — dealer, midnightmarine.net
Fuel filter carbureted engines (Rochester 2-/4-barrel, MerCarb 2-barrel): filter inside carburetor fuel inlet	Mercury 53336T	“For use on MerCruiser 3.0L, 4.3L, 5.0L , 5.7L and 7.4L engines with Rochester 2- and 4-barrel and MerCarb” — manufacturer, mercurymarine.com
Fuel filter MPI engines with Gen III cool fuel module (filter page: 2004 and newer; kit page: 2002 and newer)	Mercury Quicksilver 35-8M0093688	“Fits 2004 and newer Mercury/MerCruiser sterndrive and inboard engines with Gen III (3) fuel cooler” — manufacturer, mercurymarine.com
Raw-water impeller Bravo sterndrive and inboard engines with engine-mounted sea water pump (2-piece plastic pump per Mercury)	Mercury Quicksilver 47-59362Q01 Also: Sierra 18-3087	“For MerCruiser engines with engine mounted, 2-piece, plastic sea water pumps.” — manufacturer, mercurymarine.com · “Water Pump Impeller 59362Q01” — dealer, midnightmarine.net
Thermostat standard cooling, 160 F	Mercury Quicksilver 807252Q5	“Thermostat 807252Q5 is for V6 and V8 MerCruiser engines ... made by General Motors (model year 1987-2016)” — manufacturer, mercurymarine.com
100-hour maintenance kit (oil filter, fuel filter, drive seals) MPI engines with Gen III cool fuel module (2002 and newer)	Mercury 8M0147053	“Mercury 8M0147053 MerCruiser 5.0L, 5.7L, 6.2L, 8.1L and 8.2L V8 MPI ... 100 Hour Maintenance Kit” — manufacturer, mercurymarine.com

From the engine maker’s parts lists, or two independent parts dealers. Numbers change with serial number and get superseded — check yours before ordering.

Model sources (7)

hmy.com · bertram31.com · bertram31.com · bertram31.com · bertram31.com · bertram.com · bertram31.com

Own a Bertram 31 Flybridge Cruiser? Add her to your boats, then share a photo — names, numbers and faces are blurred before it appears.

Other Bertram models

Researched from public sources. Spotted an error? The sources are listed above — tell us what they got wrong.